The effects of ribavirin, a potent inhibitor of RNA and DNA virus replication, on the generation of primary plaque-forming cell (PFC) responses in vivo has been investigated. Intraperitoneal administration of ribavirin 1 day after immunization of C3H/HeJ mice with sheep erythrocytes (SRBC) suppressed splenic IgM and IgG PFC responses by 60 to 90%. Primary IgM PFC responses of C3HeB/FeJ mice to bacterial lipopolysaccharides (LPS), a T-independent antigen, were also inhibited to a similar extent. Inhibition of splenic PFC responses, without significant reduction in nucleated cell recoveries, was dose dependent between 0.5 and 4 mg ribavirin. Varying the time of treatment determined that optimal inhibitory activity occurred when ribavirin was administered simultaneously or 1 day after antigen. Kinetic analysis of PFC responses in ribavirin-treated mice revealed that suppression did not result from a delay in development since reduced numbers of PFC were found at all times after immunization. Ribavirin-treatment after primary sensitization in vivo also impaired the capacity of spleen cells to develop secondary PFC responses in vitro, indicating that ribavirin also inhibited memory cell generation.