Modifications of the immune response have been reported for a number of antimicrobial agents following both in vivo and in vitro experiments. Present results were obtained from in vitro incubated human polymorphonuclear cells (PMN) and monocytes treated with meropenem, a novel carbapenem antibiotic. Only the highest concentrations of meropenem (50-200 microg/ml) significantly reduced the phagocytic activity and unstimulated superoxide release of neutrophils after 1 h of incubation. Moreover meropenem (100 and 200 microg/ml) reduced PMA-stimulated superoxide release by PMN after 1 h of incubation. Only the highest concentration used (200 microg/ml) was found to reduce significantly superoxide release by PMA-unstimulated and -stimulated PMN incubated for 2 h. Meropenem did not affect some of the PMN functions studied (killing of Candida albicans, chemotaxis and glucose consumption) over a broad range of concentrations (5, 10, 20, 50, 100, 200 microg/ml). The leukocyte viability did not change even at the highest antibiotic concentration used, as showed by the trypan blue exclusion test. The LPS-induced release of IL-1alpha, IL-6 and IL-8 from isolated monocytes was not impaired by meropenem (100 and 200 microg/ml), that significantly reduced TNFalpha stimulated by LPS, after 4 h of incubation. In conclusion our data suggest that therapeutically relevant concentrations (5-20 microg/ml) of meropenem did not modify substantially the viability and the functions of human leukocytes.