Niridazole, an anti-parasitic drug, suppresses manifestations of delayed hypersensitivity and retards allograft rejection in laboratory animals. We investigated the immunosuppressive effects of the standard antihelminthic regimen of niridazole (25 mg/kg/day for seven days) in five patients with schistosomiasis. Although 15-minute skin reactions to schistosomal antigens remained unchanged, niridazole reduced or ablated positive 48-hour skin reactions to tuberculin (PPD), mumps and schistosome antigens in all patients tested six and 15 days after therapy began. Complete recovery of delayed dermal hypersensitivity was observed by 114 days. PPD-induced lymphocyte transformation was severely depressed in three and partially depressed in two of the five patients. Suppression was observed at either six or 15 days (or both) after beginning of treatment, and complete recovery at 114 days. It is concluded that therapeutic doses of niridazole suppress delayed hypersensitivity and antigen-induced lymphocyte transformation in man.