In vitro antibacterial activity of the new penem FCE 22101 was tested against blood culture isolates from 1,374 consecutive patients in a major tertiary care referral center in Saudi Arabia. Many of the isolates were significantly more resistant to commonly used beta lactams. Of the 809 members of Enterobacteriaceae tested all but 14 isolates of Enterobacter were susceptible to FCE 22101 with an MIC range of 0.12-8.0 micrograms/ml. It inhibited all the 183 isolates of Staphylococcus aureus, both methicillin-sensitive and methicillin-resistant. Ninety percent of coagulase-negative staphylococci and enterococci were susceptible to this new penem. All isolates of Acinetobacter, CDC group VE-2 and Brucella melitensis from blood were inhibited by FCE 22101. However, it was ineffective against pseudomonads. Of the nine other beta lactam drugs tested for comparison, only imipenem was found to be more inhibitory, at lower MIC values, than FCE 22101.