Inosiplex has shown antiviral activity in tissue culture, animal models and human studies through augmentative of immune responses. 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 administratration of the inosiplex in relation to that of antigen. Therefore, this study was undertaken to assess the effect of inosiplex in mice. Mice were i. p. sensitized with a dose of 10⁷ sheep red blood cells and challenged with a dose of 10⁸ SRBC thereafter. Skin reactions were read at 3hr (Arthus) and 24hr (delayed reaction). Inosiplex (200mg/kg/day) was administered i. p. for 4days before or for 3days after sensitization. Both cellular and humoral immune responses were evaluated by footpad skin reaction and splenic plaque-forming cells and hemagglutination. The effect of inosiplex on recovery of Salmonella typhimurium, an intracellular bacteria, from infected mice spleen was also studied. Inosiplex (200mg/kg) was administered i. p. one day before or after infection with bacteria. Mice were sacrificed 2days after infection and recovery of bacteria from spleen was performed by measurement of colony forming units. Inosiplex increased cellular immune response and plaqueforming lymphocytes to SRBC only when inosiplex was given after sensitization but did not when given before sensitization. Inosiplex decreased the recovery of S. typhimurium from infected mice spleen when given after infection, but increased the recovery when given before. These results suggest that inosiplex stimulate the efferent arc of the immune response and may even block the afferent arc, and that inosiplex exerts significant therapeutic benefit in microbial infection, but does not exert prophylactical benefit.