The imipenem/cilastatin constitutes a broad spectrum beta-lactam antibiotic formulation, especially used in pre and post-operatory treatments for transplanted or drug-immunosuppresed patients. The effect of the dose and the duration of the treatment with imipenem/cilastatin on some parameters of natural immunity in BALB/c mice were examined. The treatment by intraperitoneal route with 1 or 2 g/70 kg/day during 7 days did not alter significantly the parameters tested, whereas the greater dose used (4 g/70 kg/day) had an inhibitory effect on peritoneal cell counts and phagocytic activity, as well as it caused an increase on IL-1 production and natural killer activity. The greater stimulating effect of innate immunity was obtained with the lowest imipenem/cilastatin dose used (0.5 g/70 kg/day). Since this antibiotic apparently does not impair the studied innate immune responses at 1 or 2 g/70 kg/day, it seems to be especially suited for the therapy of systemic bacterial infections in immunocompromised patients.