Others are hormone "mimics" -- industrial chemicals or factory byproducts which confuse the body because they are chemically similar to natural hormones.
These pollutants are often found in very low concentrations, so until recently no equipment could detect them. But the first nationwide survey in 1999 and 2000 found hormones in about 37 percent of the streams tested.
Many scientists are concerned that people, as well as other animals, might be affected.
"It's not good news that there's something that feminizes male fish in your water," said Gina Solomon, a senior scientist at the Natural Resources Defense Council.
But the Environmental Protection Agency has not set standards for many of these pollutants. Because of this, many drinking-water plants make no special efforts to remove them.
Washington area drinking-water providers said they did not believe that the pollutants presented a problem to their customers.
"As more evidence shows up of other life forms changing, we need to look at the treatment process to make sure it protects the humans," said Tom Jacobus, general manager of the Washington Aqueduct, which supplies the District, Arlington County and Falls Church. "Right now, we think it does."
But the new concerns about cancer in West Virginia underscore how much uncertainty still surrounds these chemicals.
A recent survey of cancer in Hardy County -- where some residents get drinking water from the South Branch -- found rates of cancer of the liver, gallbladder, ovaries and uterus that were higher than the state average.
All four cancers can in some cases grow faster in the presence of estrogen or chemicals that mimic it, cancer experts said. That potential hormone connection made researchers think of the area's abnormal bass.
"It is at least theoretically possible that those two concepts are worth thinking about side-by-side," said Alan Ducatman, chairman of the Department of Community Medicine at West Virginia University.
Scientists in West Virginia are quick to urge caution about their research. For one thing, no similar cancer rates have been discovered downstream in Maryland or Virginia.
For another, the population of Hardy County is so small -- about 13,000 -- that it's a poor sample from which to discern cancer trends. What appears to be a higher-than-average rate of the disease could be a statistical fluke, scientists said.
"It's not so much we've got answers," said Pat Colsher, director of a state cancer-data clearinghouse called the West Virginia Cancer Registry. "It's that we've got some questions."
So far, the scientists' concerns have generated little public outcry in rural Hardy County, where many work in the poultry industry. Phoebe Heishman, publisher and editor of the weekly Moorefield Examiner newspaper, said that when people hear about the abnormal bass, "It's just like, 'So?' "
"There's no way that we have drawn a direct line from fish to cancer," Heishman said. "Until that happens, there's no way that people are going to get upset about it."
But a few people are worried. Jan Hawse, an employee at a middle school, said she was alarmed by news about the bass and switched from tap to bottled water.
Hawse said she thinks about her father, Robert Hartman, who died five years ago of cancer that probably began in his gallbladder or bile duct.
She wonders now whether something really is in the water.
"If that is the case, then I'm afraid it's too late for me," Hawse said. "I've lived here too long."
