A new penem antibiotic, Sch 34343, was shown to be active against a large number of Gram-positive bacteria. The drug inhibited penicillinase-positive and -negative staphylococci equally well, being five times more active than cefamandole and ten times more active than methicillin. Most methicillin-resistant staphylococci were inhibited by concentrations between 0.25 and 4 mg/l, but a small group of highly resistant strains were observed. Sch 34343 was eight times less active than ampicillin and penicillin G, but as active as piperacillin against enterococci. The drug showed excellent activity against various streptococci. Sch 34343 was as bactericidal as flucloxacillin, ampicillin and penicillin G against staphylococci, enterococci and streptococci, respectively, in killing kinetic tests. Enterobacteriaceae susceptible to third-generation cephalosporins were approximately five times less susceptible to Sch 34343, but MICs were far below the susceptibility breakpoint. Sch 34343 was equally active against Citrobacter and Enterobacter strains that were highly resistant to third-generation cephalosporins and to aztreonam. Together with thienamycin, this drug seems to be a good alternative for the treatment of infections caused by bacteria resistant to third-generation cephalosporins and to aztreonam.