The clinical need for agents to modify immune response in the treatment of viral infection has lead to an increased interest in cellular and biochemical mechanisms regulating the immune response and to the development of a variety of biological and chemical substance with immunomodulatory activity. Inosiplex has shown antiviral activity in tissue culture, animal models and huamn studies through augmentation of immune response. However, the effect of inosiplex on immune response in animal has not been extensively analyzed, and the effect of inosiplex on immune response has been paradoxical depending on the time of administration of inosiplex in relation to that of antigen. Therefore, this study was undertaken to assess the effect of inosiplex on the immune response to sheep red blood cells(SRBC) in normal and viral infected mice. Inosiplex increased cellular immune response and plaque forming lymphocyte response to SRBC, decreased the recovery of S. typhimurium from infected mice spleen, and restored the depressed cellular immune response by measle and newcastle disease virus infections. All of the above results were observed only when inosiplex was given after immunization but did not when given before immunization. These results indicate that inosiplex stimulate the efferent are of immune response and may even block the afferent are, and suggest that inosiplex is a very promising drug in therapy of many viral infections.