Aldicarb, a pesticide that is applied to the soil in many parts of the United States and that often finds its way into ground-water supplies, has been found to weaken the immune systems of mice exposed to extremely low levels of the substance in their drinking water.

The immune system protects the body against infection by viruses, bacteria and other disease-causing agents. The affected mice have not yet been tested to be sure that their disease resistance is lowered, but it is generally accepted that weakening the immune system makes an individual more likely to catch infections and makes the infections more severe and longer lasting.

Last month, California officials ordered grocers and wholesalers to destroy millions of watermelons after the illnesses of 180 people in four western states and Canada were linked to aldicarb residues.

Aldicarb, sold under the brand name Temik, is manufactured only by Union Carbide in the Institute, W. Va., factory that accidentally released an aldicarb precursor chemical, aldicarb oxime, into the atmosphere this month.

The aldicarb oxime leak at the Institute plant sickened 142 people. That chemical is treated with methyl isocyanate (MIC) to make aldicarb. (MIC is the chemical whose fumes killed more than 2,000 people when it leaked from a Union Carbide plant in Bhopal, India, in December.) It cannot be said whether aldicarb oxime would produce the same immune-suppressing effect as aldicarb.

The aldicarb levels tested for immune-system effects were below those permitted in water by the Environmental Protection Agency and comparable to levels found many times in well water in agricultural areas of several states and Canada.

The University of Wisconsin researchers who tested the chemical were surprised to discover that the lower the dose tested the greater the effect on the immune system.

"It seems crazy, I know. This was completely unexpected and, frankly, we can't explain it," said L. John Olson, an immunologist who led a seven-member research team at Wisconsin's Madison campus.

Olson said research is planned to try to learn the mechanism that produces a dose-response relationship opposite to that known for almost every other toxic substance.

In much higher doses, aldicarb behaves like a conventional toxic substance. It disrupts communication between nerve cells by binding to and disabling a natural substance needed for one nerve cell to relay its signal to another. As a result, larger and larger doses produce more nerve disruption. Low doses produce tremors and blurred vision; higher doses cause paralysis; the highest doses cause fatal coma.

The lowest doses that produce nerve effects, however, are far higher than those that produced immune-system effects.

In the experiments, healthy mice were given a normal diet except that their water was laced with aldicarb at specific low concentrations ranging from 1,000 parts per billion (ppb) to 10 ppb. EPA standards for drinking water allow up to 10 ppb. Inspections in Wisconsin, however, have found concentrations up to 100 ppb in drinking water.

After 14 days, the mice were injected with blood cells from sheep, a foreign protein commonly used to stimulate the immune system into destroying the alien blood cells. Samples of mouse blood were then removed and examined for the number of cells of a specific type that produces antibodies, the molecules that attack foreign proteins.

The scientists found that all the mice had fewer such cells than mice drinking plain water and that the reduction was greater the lower the aldicarb level. The mice drinking aldicarb at 10 ppb had only 46 percent as many antibody-producing cells as normal mice.

Startled by the findings, the Wisconsin group repeated the tests and added a still lower aldicarb dose, 1 ppb. The 1 ppb group had only 38 percent as many antibody-producing cells as normal. Tests are under way using still lower doses.

"We have not yet tested such mice to see if there is a corresponding decrease in resistance to infections and disease," said Ronald Hinsdell, another team member. "It's too early to draw any inferences about human health risks."

Aldicarb is used to kill nematodes, tiny worms that feed on plant roots. Approved for use on cotton, potatoes, sugar beets, peanuts and citrus crops, it has been found in ground water in potato-growing areas of Wisconsin, parts of Canada and citrus-growing areas of California and Florida.