More than two-thirds were contaminated with a bacterium called yersinia enterocolitica.
"This bug can cause fever and abdominal pain. And even more troubling, the vast majority of the yersinia bacteria that we found were resistant to one or more commonly used antibiotics," said Jamie Kopf of Consumer Reports.
Consumer Reports also found a few pork samples were contaminated with other bacteria that can also be harmful, including salmonella and staphylococcus And, again, some of the bacteria were resistant to certain antibiotics.
"Antibiotic resistance is worrisome because it can lead to infections in humans that are more difficult to treat," Kopf said.
On hog farms, healthy pigs are commonly given low doses of antibiotics to prevent infections and promote growth. That can accelerate the development of drug-resistant bacteria.
A second Consumer Reports test of 240 pork samples found about 20 percent had traces of the drug ractopamine, which is used in pigs to promote growth and make meat lean.
Smithfield, a major pork producer, said ractopamine is a "safe and effective Food and Drug Administration approved feed supplement that has been widely used in the hog farming industry for many years."
"The levels we found were well below the limits set by the FDA. But Consumers Union believes that it should be banned because there isn't enough evidence that it's safe for humans," Kopf said.
So Consumer Reports recommends buying pork raised without antibiotics and ractopamine. Look for meat labeled "certified organic."
And it's important to cook pork thoroughly to kill any possible bacteria. Whole pork chops and pork tenderloin should be cooked to 145 degrees. Ground pork needs to reach 160 degrees.