In a previous study, we demonstrated that parathion suppressed both the primary IgM and IgG response to sheep erythrocytes (SRC) in inbred and outbred mice (G. P. Casale, S. D. Cohen, and R. A. DiCapua, 1982, toxicologist 2, 94). Suppression occurred after a dosage which produced cholinergic effects but was absent after a lower dosage which did not produce cholinergic signs. This information suggested that immunosuppression might be mediated indirectly as a result of toxic chemical stress. The present study evaluated the relationship between the anticholinesterase action of parathion, malathion, and dichlorvos (DDVP) and their effects on the primary humoral response to SRC. Male C57Bl/6 mice were given a single dose of parathion (16 mg/kg, po), malathion (720 mg/kg, po), or DDVP (120 mg/kg, po) 2 days after immunization with SRC. Two days later, tissues were removed for cholinesterase (CHE) assay and enumeration of splenic antibody-forming cells (PFC). All three compounds produced moderate to severe cholinergic poisoning. DDVP produced cholinergic signs beginning 1/2 hr after dosing and lasting 1/2 to 1 hr. This profile was associated with a rapid but transient inhibition of brain CHE activity. In contrast, malathion and parathion produced prolonged cholinergic poisoning (4 to 7 hr) and prolonged suppression of brain CHE activity. All three compounds suppressed the primary IgM response. However, when they were given as multiple lower doses, none of the compounds suppressed the primary IgG response. These latter treatments produced no cholinergic signs. The cholinomimetic agent, arecoline (65 mg/kg, ip) produced a short-lived cholinergic crisis but no IgM suppression. Sustained-release arecoline produced prolonged cholinergic poisoning (3 to 5 hr) and reduced the number of IgM PFC to 50% of control. These results demonstrated that organophosphate-induced immunosuppression was associated with severe cholinergic stimulation. The immunosuppression may result from direct action of acetylcholine upon the immune system or it may be secondary to the toxic chemical stress associated with cholinergic poisoning.