Mice were submitted to various immunologic tests at different times after a single intravenous (IV) injection of azimexon or tuftsin in order to determine the mode of action of these chemically defined immunomodulators. Azimexon, (BM 12,531) an aziridine derivative, potentiated antibody responses to both thymus-dependent (TNP-KLH) and thymus-independent (TNP-LPS) antigens and DTH reaction to oxazolone when injected at least 1 day before the antigen. It activated macrophages, rendering them cytostatic for tumor cells, but depressed ADCC activity of spleen cells directed against antibody-coated CRBC. Tuftsin, a basic tetrapeptide, potentiated antibody response to TNP-KLH when injected at least 3 days before the antigen. The response to TNP-LPS was stimulated on days 1 and 3, but was slightly depressed on day 7. It rendered macrophages highly cytostatic for tumor cells but, as observed with azimexon, the activation process required 7 days to develop. ADCC was enhanced throughout the period of observation.