Biapenem is a new carbapenem antibiotic with high stability to human renal dehydropeptidase I. Its in vitro activity was compared with that of imipenem, meropenem, ceftazidime, ceftriaxone, piperacillin and gentamicin against a total of 650 recent clinical isolates. MICs were determined by a standard agar dilution procedure and all isolates were tested at two inocula (10(4) and 10(6) cfu). Biapenem inhibited 90% of isolates of Escherichia coli, Klebsiella spp., Proteus mirabilis, Proteus vulgaris, Morganella morganii, Providencia stuartii, Enterobacter spp., Citrobacter freundii, Serratia marcescens, Salmonella typhi, Shigella sonnei and Yersinia enterocolitica at < or = 2 mg/l and was as active as or two- to four-fold more active than imipenem against all these species, with the exception of Serratia marcescens, against which imipenem was two-fold more active. Biapenem was two-fold more active than imipenem against Pseudomonas aeruginosa (MIC90 4 mg/l) and had activity similar to that of imipenem against the Bacteroides fragilis group (MIC90 0.5 mg/l) but was two-fold less active than imipenem against methicillin-susceptible Staphylococcus aureus (MIC90 0.06 mg/l) and was, like imipenem, inactive against methicillin-resistant Staphylococcus aureus.