Inosiplex is a complex of inosine and N, N-dimethylamino-2-propanol-p-acetamidobenzoate (DIP-PACBA) in a molar ratio of 1 3. The activity of Inosiplex and the constituents of Inosiplex, inosine and DIP-PACBA, on the immunocompetence of the host, and against experimentel virus infections, was studied, In addition, the effect of the ratio of inosine to DIP-PACBA on these activities was examined. DIP-PACBA inhibited the growth of herpes simplex virus (HSV), cytomegalovirus, vaccinia virus and influenza virus (INFV) in vitro, but the activity was less potent than that of Inosiplex. Inosine did not inhibit the growth of these viruses. DIP-PACBA increased the survival rate of HSV-infected mice slightly, but the activity was obviously less potent than that of. Inosiplex. No effect of inosine was observed. Inosiplex also strongly inhibited secondary infections due to INFV, while neither inosine nor DIP-PACBA showed such activity. Inosiplex potentiated the function of both lymphocytes and phagocytes. Neither inosine nor DIP-PACBA showed such activity. The most potent immunpotentiating activity was shown for complex of inosine and DIP-PACBA with a: molar ratio of 1: 3. Since the activity of Inosiplex as a potentiator of host defenses was not shown by the individual constituents, inosine and DIP-PACBA, it is suggested that the activity may be exerted by the formation of a complex of the constituents.