FCE 22101 is a penem antibiotic with broad in-vitro activity similar to that of imipenem, although less active against Pseudomonas aeruginosa. Combinations of FCE 22101 with ciprofloxacin or gentamicin against methicillin-resistant Staphylococcus aureus and methicillin-resistant Staph. epidermidis resulted in addition or indifference by the chequerboard method. The combination of FCE 22101 and gentamicin against Streptococcus faecalis was usually additive. Against Enterobacter cloacae FCE 22101 had MICs of 16 mg/l whilst in combination with gentamicin (0.25-0.12 mg/l) the MICs were reduced to less than 2 mg/l. Ciprofloxacin and FCE 22101 showed only addition against Enterobacter spp. similar results were obtained with the combinations of FCE 22101 and gentamicin or ciprofloxacin tested against Citrobacter freundii. With Serratia spp. FCE 22101 and gentamicin showed synergy, but FCE 22101 and ciprofloxacin showed indifference. Similar results were obtained with strains of Escherichia coli and Klebsiella, Proteus, and Providencia spp. FCE 22101 plus gentamicin, or aztreonam, against Ps. aeruginosa usually showed indifference, but with ciprofloxacin addition was the rule. In general the combination of FCE 22101 with other agents resulted in an additive rather than a synergistic effect.