The in-vitro activity of the penem Sch 34343 was assessed in comparison with third-generation cephalosporins, aztreonam and imipenem. Sch 34343 was active, with MICs generally 2 mg/1 or less, against a broad spectrum of organisms amongst which were staphylococci (including methicillin-resistant strains), streptococci (though enterococci were less susceptible with Sch 34343 MICs mostly 4–8 mg/1), anaerobes (including the Bacteroides fragilis group), Haemophilia influenzae and Neisseria gonorrhoeae . It was also active against the Enterobacteriaceae though Enterobacter cloacae and Serratia spp. were somewhat less susceptible than other members of this group. Sch 34343 lacked useful activity against most species of Pseudomonas (including Ps. aeruginosd ) but was active against Ps. acidovorans and some isolates of Ps. cepacia . Sch 34343 showed a high degree of resistance to plasmid-dctermined and most chromosomally-determined β -lactamases; however, it was hydrolysed rapidly by enzymes from some isolates of Aeromonas hydrophila and more slowly by enzymes from Ps. maltophilia and Bact. melaninogenicus subspecies intermedius . We did not observe any reduction in sensitivity to Sch 34343 for most β -lactamase-producing organisms, including strains of Enterobacter spp. and Citrobacter freundii that were resistant to most other β -lactams.