A study of Agriculture Department records found the meat safety system plagued by delays.
At some plants, testing stopped for months at a time before being completed. In other cases, the department waited weeks to take corrective action at plants that had clearly flunked, said the report released by Public Citizen and the Government Accountability Project.
The report accused USDA of operating under a "don't look, don't find policy" that is "fundamentally deceiving the public with false reassurances" about the safety of meat.
Elsa Murano, USDA's undersecretary for food safety, said the testing system "is continuously being reviewed, evaluated and improved" and that the department is "aggressively targeting" plants that fail to control bacteria.
The groups said the findings raise questions about testing data that the department has presented as evidence of reduced salmonella contamination at plants.
"Companies were failing these tests and USDA was allowing them to continue to put out meat stamped inspected and approved for extended periods of time, and they're still doing it," said Carol Tucker Foreman, director of the Consumer Federation of America's Food Policy Institute.
In addition to being a health hazard itself, the presence of salmonella is considered by USDA to be an indication of sanitation problems in meat plants. The Clinton administration started the testing program after a 1993 E. coli outbreak linked to tainted burgers killed four people and sickened hundreds.
Salmonella can cause serious and sometimes fatal infections in children, the frail and the elderly. Healthy people infected with salmonella often experience fever, diarrhea, nausea and vomiting.
Under USDA's rules, ground beef processors are considered to have failed the tests if six of 53 meat samples test positive for the bacteria. But even if the first six samples are positive, USDA doesn't consider a plant to have failed until all 53 samples are completed — a process that can take months, the report said.
At one Texas plant cited by the report, 16 weeks passed after the sixth positive sample was discovered before any corrective action was required, the report said. By the time all 53 samples were collected, 25 were positive for salmonella. At an Arkansas plant, it was 19 weeks after the sixth positive sample until the department took action.
Last year, the Bush administration abandoned a court battle with the meat industry over the government's authority to close plants that repeatedly failed the tests.
The testing standards are based on average contamination rates in the 1990s and vary with the type of meat and poultry. The meat industry says they are not scientifically based.
USDA long has credited the testing for its reported drops in salmonella levels on meat and poultry. Industry officials say the decline is due to improvements they have made in plant sanitation systems.
Last year, 2.8 percent of ground beef tested positive for salmonella bacteria, compared with 3.3 percent in 2000 and 6.4 percent in 1998, according to the Agriculture Department. Murano said she is confident that the data are reliable.
The department recently announced that it would start requiring beef-grinding plants to have at least one antimicrobial treatment for beef — or else buy their meat from a slaughterhouse that does.