A one-hour exposure to one of the chemicals, triphenyltin, reduces the tumor-killing ability of natural killer cells by 50 to 60 percent, biochemist Margaret Whalen of Tennessee State University in Nashville, who led the study, reported.

And the cells did not work properly again for six days after the exposure, Whalen and graduate student Sharnise Wilson told a meeting of the American Chemical Society in Orlando.

"The results indicate that brief exposures to triphenyltin can cause persistent suppression of human immune system function," Whalen said.

Whalen's team did the study in laboratory dishes, and now she says they are going to test agricultural workers who have used the pesticide to see if they have any in their blood.

"It's hard to know what real-life levels for phenyltins are," she said. "It is not a chemical that the majority of us are going to be exposed to."

A related group of chemicals called tributyltins have similar effects on natural killer cells, Whalen said.

Natural killer cells are one of the arsenal of white blood cells called lymphocytes. They are a first line of defense in the human immune system, targeting and killing cells that do not carry a flag, called the major histocompatibility complex, that marks them as "good guys".

Whalen decided to test the effects of various chemicals on these cells, and chose from a list of compounds found to cause cancer and hormone disruption in rats and mice.

Phenyltins have been found to contaminate water, sediment, and fish.

"I took lymphocytes from normal human donors ... and exposed them in a test tube to varying levels of the compound," Whalen said in a telephone interview.

"The compound limited one immune function of the natural killer cells which is to kill tumor cells. What was startling to us was that if we removed the compound, the inability to kill tumor cells continued."

Whalen said her team will try and test farm workers and will also do lab tests to see if the body can get around the damage caused by the pesticides.

"Is it possible in any way to reverse this?" she asked. Perhaps signaling chemicals called interleukins can have an effect, she believes.

Of more than a dozen chemicals she screened, only two groups, triphenyltins and the related tributyltins, had this effect on the natural killer cells, Whalen said.

"These chemicals were made to replace things that are more toxic," she said. "This was considered an improvement on things that were used in the past."