In order to study the comparative cross-reactivity of human interferon (Hu-IFN) with cynomolgus monkeys, the effect of Hu-IFN on the immune responses of monkeys was assessed by measuring lymphocyte transformation induced by phytomitogens. The in vivo administration of Hu-IFN or its placebo to monkeys resulted in transient neutrophilia, but no significant alteration of the kinetics of T lymphocytes in the peripheral blood. Lymphocyte proliferation of Hu-IFN-injected monkeys showed a significant reduction depending on their serum Hu-IFN titers, whereas no significant decrease was observed in monkeys injected with placebo. The in vitro incubation of monkey lymphocytes with Hu-IFN before or simultaneously with the addition of phytomitogens caused a substantial decrease in the lymphocyte proliferative responses in a dose-dependent manner. However, the inhibitory effect was no longer observed when Hu-IFN was added 24 h after incubation of lymphocytes with phytomitogens. Hu-IFN also suppressed the blastogenic responses of lymphocytes pulsed with phytomitogens before the addition of Hu-IFN. Addition of peritoneal macrophages pretreated with Hu-IFN failed to exert any significantly different effect on the lymphocyte proliferation compared to untreated ones. Taken together, the inhibitory effect of Hu-IFN seemed to act directly upon the early phase of lymphocyte proliferation induced by phytomitogens. These findings indicate that cynomolgus monkeys were susceptible to Hu-IFN both in vitro and in vivo with respect to the inhibitory effect on lymphocyte proliferation and could serve as a useful experimental model to study the effect of Hu-IFN on the immune responses.