Twenty-nine individuals with chronic health complaints following exposure to chlorpyrifos were compared with 3 control groups (i.e., 1 positive and 2 negative) with respect to the following: (1) peripheral lymphocyte phenotypes; (2) autoantibodies (nucleic acids and nucleoproteins, parietal cell, brush border, mitochondria, smooth muscle, thyroid gland, and central nervous system/peripheral nervous system myelin); (3) mitogenesis to phytohemagglutinin and concanavillin. The data revealed an increase in CD26 expression, a decrease in percentage of CD5 phenotype, decreased mitogenesis in response to phytohemagglutinin and concanavillin, and an increased frequency of autoantibodies. The alterations in these peripheral blood markers were unaffected by medications, age, sex, or season. The authors concluded that chronic exposure to chlorpyrifos causes immunological changes.