The in vitro effects of ISO on the immune responses of cancer patients were investigated. Forty seven patients with primary tumors (26 lung carcinoma, 14 breast adenocarcinoma, 7 melanoma) were studied. Concanavalin A (ConA)-induced lymphocyte proliferation, natural killer cell (NK) activity, and monocyte chemotaxis were measured. In 40 of the 47 patients (85%), ConA-induced lymphocyte proliferation was depressed; NK activity was depressed in 32 (68%), and monocyte chemotaxis was found to be depressed in 36 (77%). For in vitro studies, an optimum concentration of ISO (100 micrograms/ml per 10(6) cells) was used to treat peripheral blood mononuclear cells. In the presence of ISO, all three parameters were restored to normal or near normal levels in those that were depressed. Under these preincubation conditions in vitro treatment of mononuclear cells from the peripheral blood of normal individuals with ISO had no effect on their activities in the three assays. Similar effects on these three immune parameters were observed when 24 h supernatants obtained from patients' mononuclear cells pretreated with ISO were employed in these assays.