The specter of another bioterrorism attack is adding new pressure on Bayer Corp. to quit making an antibiotic for chickens that it manufactures in the Kansas City area.

Bayer makes Cipro, the antibiotic doctors turned to when a terrorist sent out anthrax-laced letters that sickened 13 persons and killed five. Bayer also makes Baytril, the animal version of Cipro, which is used to treat respiratory problems in chickens and turkeys.

For more than a year, Bayer has been fighting an attempt by the Food and Drug Administration to ban the use of fluoroquinolones, which include Baytril, for poultry. The FDA says that since 1995, when Baytril was approved for poultry, there has been a dramatic increase in the percentage of cases in which people developed food-borne illnesses that resisted treatment by Cipro and other human forms of those drugs.

Bayer disputes arguments that growing resistance to these antibiotics is tied to their animal antibiotic. Bayer asked the FDA to drop the matter or for the issue to be determined by an administrative law judge. The FDA granted Bayer's request for a hearing.

Some members of Congress and medical and consumer groups, which support the ban, are pressuring Bayer to voluntarily pull the drug. They point to the potential of future bioterrorism attacks in their arguments.

Sen. Harry Reid of Nevada recently sent Bayer a letter saying that if the company does not voluntarily withdraw the drug, he will introduce legislation to phase out Baytril's use in poultry.

"Like all Americans, the recent anthrax attacks (have) made me more keenly aware of the importance of ensuring the viability of drugs like Cipro for treating anthrax and other numerous diseases," Reid said in his letter to Bayer Chief Executive Helge Wehmeier.

"Although no evidence yet indicates that Baytril use in poultry threatens our ability to treat anthrax, FDA has concluded that such use is already making Cipro less effective in treating other disease-causing bacteria, such as those that cause severe food poisoning."

The debate has led some restaurant companies, like McDonald's Corp., to stop buying chicken that has been treated with fluoroquinolones, and some chicken processors to decide not to use the drug on their flocks.

"We got a lot of input on this issue from outside experts," said Walt Riker, a McDonald's spokesman. "We did this quietly about a year ago. I guess we were ahead of the curve as it turns out."

Officials with Bayer Animal Health in Johnson County say the FDA's evidence is flawed. Moreover, they said any suggestion that use of Baytril would make Cipro ineffective in treating anthrax is bogus. It would be bad for science, they contend, to bow to pressure based on sentiment instead of science.

"We have always maintained our decision would be based on the science," said Dennis Copeland, a veterinarian and director of government-industry relations for Bayer Animal Health. "It would be bad precedence for us not to pursue this when we feel the science is on our side. It would be wrong."

He said they would make their case before a judge later this year.

Bayer has declined to say how much revenue Baytril generates for the company, though a competitor has pegged the market, which Bayer dominates, at $15 million a year. A Bayer official said that if the drug were banned, it would have minimal impact on the company's revenues or its employment in the Kansas City area.

FDA officials agreed with Bayer that there is no scientific foundation for saying that Baytril's use in chickens will make Cipro less effective in treating anthrax. But there is plenty of evidence that Baytril's use in chickens makes human antibiotics less effective in treating illnesses people get from eating poorly prepared poultry.

"The debate as to the use of antibiotics in food-producing animals has been going on since the 1970s. Only in the past five years have we had good data that shows human health is impacted," said Linda Tollefson, a veterinarian and deputy director of the FDA's Center for Veterinary Medicine, which is seeking the ban. "Part of the delay was not that it wasn't happening, but that our tools were too crude to detect it."

An FDA advisory committee voted in 1995 to approve the use of a fluoroquinolone in chickens, but required a monitoring system to measure any increase in cases in which people got sick from bacterial infections that resisted treatment by Cipro and similar drugs.

According to the FDA, in 1995 about 1 percent of those people who were victims of Campylobacter infections had cases that were resistant to the antibiotics. In 1998, that figure had increased to 13.3 percent. By 1999 it was at 17.6 percent. In 2000 and 2001, the number of cases was at about 14 percent, Tollefson said.

Citing those data, the FDA in October 2000 announced it wanted to remove fluoroquinolones from the animal market. At that time only Bayer and Abbott Laboratories of North Chicago, Ill., had such drugs on the market.

Abbott agreed to withdraw its antibiotic immediately. A spokesman said at the time that the entire market for the drug was only about $15 million and Bayer had, by far, the largest percentage of that market.

A year ago, Bayer officials filed their own findings with the FDA. They claim the data show that the number of Campylobacter infections in humans decreased from 1996 through 2000, even as chicken consumption increased. They said data from the Centers for Disease Control and Prevention show cases of antibiotic-resistant Campylobacter decreased from 1997 to 2000.

The CDC says the total number of drug-resistant Campylobacter cases is down - they contend because chicken producers are doing a better job of keeping chicken carcasses free of bacteria. The percentage of those cases resisting antibiotics, however, remains about the same over the last few years, around 14 percent - much higher than before Baytril was approved for use.

"The FDA's concern over cross-resistance is legitimate, but how much of it comes from animals and how much from other factors?" said Copeland, of Bayer. "When we look at the evidence and consult with third-party experts, it doesn't support the conclusion that it comes from our product."

The hearing process should be completed this year, said Tollefson of the FDA. There is no appeal from the administrative judge's decision.

Supporters of the ban, including Reid, hope Bayer will yield to public pressure before that. Some of Reid's staff took Cipro when a letter containing anthrax showed up in a Senate office building.

"Using this stuff for the poultry industry is overkill when it could potentially endanger all of our lives," said Nathan Miller, Reid's press secretary. "Bayer should take an opportunity to be the good guy on this. Sometimes the bottom line is not the bottom line."