Twelve individuals who were exposed to chlorpyrifos were studied 1-4.5 y following exposure to determine changes in the peripheral immune system. The subjects were found to have a high rate of atopy and antibiotic sensitivities, elevated CD26 cells (p < .01), and a higher rate of autoimmunity, compared with two control groups. Autoantibodies were directed toward smooth muscle, parietal cell, brush border, thyroid gland, myelin, and ANA. Chlorpyrifos exposure was implicated in the immunologic abnormalities reported. The immunologic changes were similar to those reported for other pesticides.