Male fish from the upper reaches of the Potomac River are producing a blood protein normally found only in females, scientists say, seeming to confirm fears that an unknown pollutant is confusing the fish's natural hormone systems.

This discovery, made by scientists at the Virginia Institute of Marine Science, comes on the heels of the finding last year that male fish in the same area were producing eggs.

Those intersex fish have put the Potomac River's headwaters -- small streams in West Virginia farm country -- at the center of an emerging national pollution problem. Scientists say that some runoff from factories, farms and sewage plants contains chemicals that animals' bodies mistake for their own hormones, throwing off the natural signals that guide their development.

A recent experiment looked at smallmouth bass from the South Branch of the Potomac, the same species that previously had been found to be growing eggs there. Scientists were looking for another sign that something was wrong with the fish, to confirm that some male bass don't exhibit this strange trait naturally.

"It should not be normal, but, of course, we didn't really know," said Vicki S. Blazer, a biologist at the U.S. Geological Survey who has led this research.

Scientists took blood from the South Branch fish, as well as from other male bass taken from two West Virginia rivers outside the Potomac watershed. All the blood was tested for vitellogenin, a protein made in the livers of female fish and later converted into the egg "yolk" that feeds their embryos.

Male fish have the vitellogenin-making gene but don't usually have the female hormones that turn it on, scientists say. For this reason, other studies of abnormal fish across the country had used vitellogenin as a marker of abnormal development in males.

Among the Potomac fish, six of the 14 that were tested showed signs of the protein. Among the 17 fish taken from the other rivers, none tested positive for vitellogenin.

Peter A. Van Veld of the Virginia marine science institute said it was the confirmation they were looking for.

"It suggests that the fish in the Potomac River are being exposed to compounds that behave like estrogens," he said.

But what the compounds are is a mystery: Extensive water testing has so far failed to point to a particular pollutant as the cause of the problem.

Scientists say the most likely culprits could be nearby chicken farms, which produce manure full of chicken estrogen, and sewage plants, whose output can include hormones that are excreted from the human body.

Another question still unanswered: How far downriver does this problem extend? Last year, researchers found intersex fish in the main Potomac, 60 miles upstream from Washington.

This fall, U.S. Fish and Wildlife Service researchers will be looking much closer to the Washington area. They will be catching fish to test from the Monocacy River in Frederick County and the main Potomac at the District's Blue Plains sewage plant.

-- David A. Fahrenthold

Scientists are examining smallmouth bass from the South Branch of the Potomac in Moorefield, W.Va.