SHADY SIDE, MD. -- Marine scientists are using sheepshead minnows, grass shrimp and water fleas as their guinea pigs in a new study to determine the toxicity of pollution flowing into the Chesapeake Bay.

"We're getting it right from the horse's mouth," said Daniel J. Fisher, a Johns Hopkins University scientist taking part in the new study. "We're letting the species make the decisions."

The three-year, $485,000 state-funded study will test 55 major industries and 35 sewage plants located everywhere from the Eastern Shore to Western Maryland. So far, 28 facilities have been tested, including five sewage treatment plants.

"Seven of those have shown some toxicity," said George H. Harman, a division chief at the Maryland Department of the Environment, which is overseeing the so-called bioassay.

In the early 1980s, toxic materials were found to pose a serious threat in parts of the bay and its tributaries, according to the Chesapeake Bay Program of the U.S. Environmental Protection Agency.

The findings, coupled with the 1984 EPA policy statement urging states to identify toxics, spurred Maryland officials to begin the bioassay tests.

Some scientists are troubled by the initial results, but most say findings are still preliminary.

"What's alarming is that some companies are not aware that they have toxicity," said Dennis Burton, who heads the study.