A line of evidence indicates changes of the immune system in schizophrenic patients. We investigated the production of cytokines by peripheral blood mononuclear cells (PBMCs) in drug-free and neuroleptic-treated schizophrenic patients compared to healthy, normal controls. A significant reduction in interleukin (IL)-2 production was detected in untreated schizophrenic patients (-59.6%; p < .05) as well as in IL-3-like activity (IL-3-LA) production (-27.4%; p < .05) in treated patients compared to controls. No alteration was observed in IL-1 beta production. It seems that schizophrenia is associated with diminished IL-2 production, while neuroleptic treatment interferes with the capacity of immunocompetent cells to synthesize and/or release Il-3-LA. The alteration in cytokine production did not correlate with either the severity of the disorder or the serum prolactin levels.