The researchers say the findings suggest antibiotic-resistant bacteria may persist in the poultry industry after the use of the antibiotics, known as fluoroquinolones, has stopped and may contaminate more poultry than previously thought.

However, one of the producers and a researcher not involved with the study said it did not show whether the amount of bacteria found presented a health risk. Whether the resistant strains were naturally present or use of the antibiotic caused the resistant strains to be present in the samples also was not clearly shown, the two said.

In 2000, the Food and Drug Administration proposed banning the two fluoroquinolones used in poultry. Abbott Laboratories of Abbott Park, Ill., agreed immediately to pull its version, Sara Flox, off the market, but Pittsburgh-based Bayer Corp. is appealing the decision and the FDA commissioner is considering the case involving Bayer's product, Baytril.

The bacteria, campylobacter, is responsible for 2.4 million cases of food-borne illness per year in the United States, according to the Centers for Disease Control and Prevention. The antibiotic-resistant form of the bacteria is especially troubling because the fluoroquinolone family of antibiotics includes the popular drug Cipro, and fluoroquinolones are a leading treatment for food poisoning from campylobacter, found mostly in raw chicken.

In February 2002, Perdue Farms Inc. of Salisbury, Md., and Tyson Foods Inc. of Springdale, Ark., stopped using fluoroquinolone antibiotics for flock-wide treatment.

The researchers said they bought chicken produced by Perdue, Tyson, and two antibiotic-free producers from Baltimore-area supermarkets between February 2003 and May 2003, buying three packages each a total of seven to eight times.

The researchers used two different methods to test for the bacteria. In one, samples were placed in a growth medium to see if colonies of the bacteria would form. In the other, the medium contained fluoroquinolone, which the researchers said enhanced the sensitivity of the testing for resistant bacteria.

Using the plain growth medium, campylobacter was detected on 84 percent of the chicken tested, and fluoroquinolone-resistant strains were detected on 17 percent. When the fluoroquinolone medium was used, resistant strains were detected on 40 percent of the chicken tested.

By brand, resistant bacteria were found in 33 percent of Tyson chicken sampled, 96 percent using the fluoroquinolone medium; and 19 percent of Perdue chicken, 43 percent using the antibiotic medium.

Levels in the two antibiotic-free producers were much lower. Resistant bacteria were found in 13 percent of samples from one producer using either testing method, and in 5 percent from the other producer using the antibiotic medium. Resistant bacteria were not found using the regular medium in that brand.

"I think the message to the consumer is that they should choose their brand of chicken carefully," said study author Lance Price. "And they should consider the use of clinically important antibiotics and antimicrobials in food animal production to be a potential threat to their safety."

Tyson noted the sample size was small and limited to one area of the country, did not measure the amount of bacteria present, and included species of campylobacter that may be naturally resistant to the antibiotics without having been exposed to them.

Price said if natural immunity was responsible, "you would expect all the products to come out the same, but that's not the case."

Perdue said it did not have any information on the origin of the samples, and "cannot comment on the conclusions regarding antibiotic resistance."

Perdue said less than 1 percent of its flocks receive any antibiotics, which are limited to the "humane treatment of ill or at-risk chickens, treating as few birds as possible, and prescribing that treatment no longer than deemed medically appropriate by a poultry veterinarian.

"Perdue does not use antibiotics for the purpose of growth promotion nor do we use any antibiotics continuously for any reason whatsoever," according to a statement the company issued.

Randall Singer, an assistant professor of epidemiology at the University of Minnesota, said the study did not show how much bacteria was present in each sample, and thus did not show whether the bacteria presented a health risk.

Singer, a member of two FDA panels on campylobacter and fluoroquinolone use in poultry, said other studies have shown the resistant strain is more successful, even when antibiotics are not present, and therefore could be more likely to be present naturally.

"Clearly, there is a background level of fluoroquinolone-resistant campylobacter, and when you want to compare retail meat products, especially on a small scale like this study, there could be many reasons," Singer said.