Our laboratory has proposed a working model which asserts that cocaine's effects on immunity are mediated by reactive metabolites generated by the cytochrome P-450 system. This metabolic pathway is normally a minor one in humans, but takes on significance when metabolism of cocaine by the P-450 system is increased, as may occur with excessive alcohol consumption (enzyme induction) or after exposure to organophosphate pesticides (esterase inhibition). Results from our laboratory demonstrate that cocaine exerts its most dramatic effects on immunocompetence when administered to mice that have been pretreated with diazinon, an organophosphate esterase inhibitor. Most notably, we observed decreases in both the splenic T-dependent antibody response to sheep erythrocytes and the splenic T-independent antibody response to DNP-ficoll and a dramatic thymic atrophy in mice exposed to cocaine + diazinon, which were not seen in mice exposed to cocaine alone. The primary objective of the present investigation was to determine whether the exposure conditions used to produce the changes noted above are also capable of causing changes in lymphocyte cell types by use of flow cytometric analysis. Administration of cocaine after pretreatment with diazinon only modestly affected splenic lymphocyte subsets, which caused a slight decrease in the number of B cells. No effect was observed in the macrophage, T-helper or T-suppressor subpopulations in the spleen. These results suggest that changes in splenocyte subpopulations induced by cocaine + diazinon cannot account for the suppression of the antibody response. In contrast, all T-cell subsets in the thymus were decreased significantly, with immature double-positive thymocytes suffering the greatest loss in cell number. These results indicate that T cells, especially immature thymocytes located in the thymus, are sensitive to effects associated with the combined treatment of cocaine + diazinon.