Continued administration of potent virally induced mouse interferon (IFN type I) preparations to suckling mice resulted in an inhibition of the primary antibody response to sheep erythrocytes (14-day-old mice). When slightly older suckling mice were immunized (17 days old), a delay (about 2 days) in the kinetics of the response was observed, but the amplitude of the response was similar in both control and IFN-treated animals. In adult mice, potent IFN preparations did not inhibit the antibody response under a variety of experimental conditions (different doses of IFN, schedules of treatment, routes of injection, times of assay). Although IFN does inhibit the in vitro antibody response, we conclude that under most experimental conditions, injection of IFN type I is not immunosuppressive in adult mice. When administered after immunization, IFN clearly enhanced the primary antibody response. It is of interest that IFN, a product of viral infection, enhances in vivo those components of the immune system that have been implicated in the host response to viral infections (i.e. delayed-type hypersensitivity, natural killer cell and macrophage activity, expression of lymphocyte membrane molecules), and, as is shown here, the primary antibody response.