Interleukin 2 (IL-2) and gamma interferon (IFN-gamma) were determined in supernatants of mitogen- and antigen-driven cell cultures from patients with hepatosplenic or intestinal schistosomiasis. Skin reactivity was tested using a panel of eight recall antigens. Results were compared with those of uninfected local controls. In both schistosomiasis groups, IL-2 activity was reduced before treatment. In less than one third of the patients, schistosomal antigens elicited detectable IL-2 activity. IFN-gamma production was reduced more severely in hepatosplenic cases, in particular after stimulation by anti-CD3 monoclonal antibodies. After anti-schistosomal therapy with praziquantel, mitogen-induced IL-2 and IFN-gamma activities became normal within 3 months in intestinal schistosomiasis, and within 6 months in the hepatosplenic patient group. Results of in vivo delayed-type hypersensitivity tests paralleled those of in vitro lymphokine production. In conclusion, evidence is presented for severe, antigen-unspecific suppression of lymphokine production and skin reactivity against recall antigens. Anti-parasitic chemotherapy is shown to reverse the impairment of cell-mediated immune responses at the cytokine level.