The influence of certain psychotropic drugs on the delayed-type hypersensitivity (DTH) response to sheep red blood cells in BALB/c mice was studied. The effects on the overall response and the induction and elicitation phases were evaluated, using two alternative dosage schedules for each agent. It was found that diazepam had no effect on the DTH reaction but the administration of imipramine, haloperidol, chlorpromazine or meprobramate all resulted in depression of the response, impairing both the induction or elicitation phases. The results indicate that psychotropic drugs may produce in vivo depression of cell-mediated immunity by different mechanisms.