Imipenem is a member of a new class of beta-lactam antibiotics, carbapenems, with a very broad antibacterial spectrum. In this work we evaluated the in vitro activity of imipenem against a variety of bacterial strains isolated from clinical specimens as well as the activity of other beta-lactam antibiotics. The results obtained with 501 bacterial strains show that imipenem is active on both gram-negative and gram-positive microorganisms isolated from different infections. The in vitro inhibitory activity is greater than that of aztreonam, cefotaxime, ceftriaxone, piperacillin, amikacin, and netilmicin, against the majority of strains tested.