A new carbapenem antibiotic, meropenem, was shown to be active against a large number of Gram-positive bacteria. The drug inhibited penicillinase-positive and -negative, methicillin-susceptible staphylococci equally well. Among the comparative antimicrobials examined, only N -fonnimidoyl-thienamycin (imipenem) was two to four times more active than meropenem. Compared with vancomycin or methicillin, meropenem was 10–20 times more active. Strains of 11 species of streptococci were highly susceptible to meropenem; the geometric mean MICs of the drug for these species ranged from 0.01 to 0.04mg/l. The agent, however, only had moderate activity against Enterococcus faecalis (mean MIC 5mg/l) and Ent. faecium (mean MIC 11.6 mg/l). Among Corynebacterium jeikeium , strains were encountered that showed susceptibility to meropenem but resistance to imipenem and other β -lactams. Strains of other corynebacteria, Rhodococcus equi, Erysopelothrix rhusio-pathiae, Listeria monocytogenes, and Bacillus spp. all were highly susceptible to meropenem (mean MICs 0.04–0.17 mg/l). Although methicillin-resistant staphylo-cocci were inhibited by concentrations of 1–2 mg/l of meropenem in agar dilution tests, such strains showed heteroresistance in population studies, as is typical for all β -lactam antibiotics. In addition, the biochemical correlate of methicillin-resistance, penicillin-binding protein 2′, showed low affinity for meropenem, similar to that for imipenem. Meropenem was as bactericidal as imipenem and comparative bactericidal antimicrobials in killing-curve experiments. Strains of Ent. faecium, C. jeikeium , and L. monocytogenes were killed at a slower rate than streptococci or staphylococci.