The effects of the two antipsychotic drugs, chlorpromazine and haloperidol, the focus of this study, on cell-mediated immunity in male ICR mice. The peripheral blood WBC count decreased significantly in both cholorpromazine and haloperidol. The absolute lymphocyte count decreased only in the haloperidol-treated groups. The absolute number of thy-1-bearing cells described in both the chlorpromazine and haloperidol groups, the most remarkable effects evidencing itrself in the booster groups of higher dosage chlorpromazine (15 mg/kg), and lower and higher-dosage haloperidol (1 mg/kg and 5 mg/kg). The absolute spleen T-lymphocyte count was decreased significantly in the chlorpromazine higher-dosage booster-dose group and the haloperidol higher-dosage (5 mg/kg) single-dose group and the haloperidol lower and higher-dosage (1 mg/kg and 5mg/kg) booster-dose group. Also, chlorpromazine and haloperidol significantly impaired the in-vitro lymphocyte response to phytohemagglutinin (PHA) and produced a negative reaction of the delayed-hypersensitivity type induced by BCG vaccination. These findings suggest that chlorpromazine and haloperidol suppress the cellular immune responses in mouse.