Meropenem, a new carbapenem, was compared with imipenem and seven other broad-spectrum antimicrobial agents against approximately 1000 clinical isolates. Meropenem was two- to four-fold more active than imipenem against Gram-negative organisms and its spectrum of antimicrobial activity was wider than those of all other drugs tested. However, imipenem was more potent than meropenem against the staphylococci, Streptococcus spp. and enterococci. Many rarely isolated organisms were more susceptible to the carbapenems than to other comparison compounds. All anaerobic bacteria were inhibited by meropenem at ≤ 8 mg/l and 50% of strains were inhibited by 0.25 mg/l. Meropenem MICs were not significantly influenced by high inocula and the drug was generally bactericidal. Strains producing various 0-lactamases remained susceptible to meropenem but some isolates producing high levels of chromosomally-mediated enzymes showed an inoculum effect only at 10 7 cfu/ml. Meropenem demonstrated antagonism with several other β -lactams against strains producing Type I cephalosporinases. Susceptibility tests performed on agar and in broth produced very similar meropenem results. Imipenem and meropenem shared a high degree of cross-susceptibility as measured by dilution test methods. Disc diffusion (10- μ g disc) regression-line correlations with meropenem MICs are reported with two possible sets of interpretive criteria, using meropenem breakpoints of ≤ 2 and ≤ 4 mg/l.