A study scheduled to be released Wednesday and obtained by USA TODAY reports that 13 of the 52 supplements (25%) purchased at various U.S. retailers contained small amounts of steroids and six (11.5%) had banned stimulants.

The study was overseen by Informed-Choice, a non-profit coalition of dietary supplements. Testing was conducted by HFL, a lab approved by the World Anti-Doping Agency and located in England.

"We were very surprised by these results," said Dave Hall, CEO of HFL.

A study by the International Olympic Committee conducted from 2000 to 2002 showed 18.8% of the 240 supplements purchased in the USA contained steroids.

Officials from Informed-Choice and HFL declined to reveal which supplements were tested or where they were purchased, but Hall said some of best-selling brands were randomly selected, mostly from mall-based stores.

"We didn't go after products that appeared to be dubious," Hall said. "We wanted to test products that were standard and mainstream, something a high school kid would be attracted to."

John Barnes, the football coach at Los Alamitos (Calif.) High, was alarmed at the results.

"It's not good if a kid can find these things in a health-food store," he said. "As a father and a coach, you think anything they'd find there would be perfectly fine, and then you find out that it's banned. This is something I'm going to start talking to my players about right away."

Of the 10 categories of supplements tested, testosterone boosters were flagged the most often. Six of the nine boosters contained androstenedione, a steroid precursor used by Mark McGwire en route to setting the single-season home run mark in 1998.

Andro was made illegal nearly three years ago.

Kelly Hoffman, executive director of Informed-Choice, said most banned substances are finding their way into products inadvertently - for instance, when manufacturers fail to properly clean equipment. She said raw materials imported from Asia, India and Eastern Europe also can be contaminated.

The Food and Drug Administration is taking on a more active role in policing supplements, including taking reports of adverse effects of the products starting Dec. 22.