Check labels and packaging to see what type of fish the oil came from, and what part of the fish was used.

Oil from anchovies or other small fry, or from wild salmon, is less likely to be contaminated.

Dr. Cooperman also pointed out that the lawsuit focused on sources of fish oil most likely to be contaminated—oil pressed from the livers, and oil taken from larger fish. (As a rule of thumb, the larger the predatory fish, the greater the contamination.) However, these aren't the supplements consumers usually pick.

Here's how to choose the best fish oil supplement:

• Find oil from fish lower on the food chain. Stay away from cod liver or shark oils. Toxins accumulate in these large, predatory fish. Instead, opt for fish oil supplements made from smaller fish rich in omega-3s, such as anchovies, mackerel, or sardines. "These fish are going to have much lower levels [of contaminants]. There will still be a little bit," Dr. Cooperman adds. "It's unavoidable. It's in the water. But these are not dangerous levels at all."

You can keep salmon on your fish list too. Even though a salmon oil supplement was named in the lawsuit (NOW Foods Salmon Oil) because of its overall PCB levels, testing found it was extremely low in the harmful dioxinlike PCBs. Salmon oil is considered safe as long as its source is wild-caught Alaskan salmon, not its farmed counterpart. Fish taken from open bodies of water tend to have lower contamination levels than ones taken from more polluted inland bodies of water.

• Take note of the origin. Fish oils taken from the fish liver are going to be higher in PCBs because the liver is the organ that deals with contaminants. You want a fish oil pressed from the whole fish or fish flesh to get lower levels of PCBs and heavy metals.

For more information, ConsumberLab.com offers a subscription report on fish oil and plans to release an updated report this summer.