The antibacterial activity of novel ${eta}$-lactamase inhibitors, 6-exomethylene penamsulfones (CH1240, CH2140), has been compared in vivo with that of sulbactam and clavulanic acid against b-lactamase producing strains. In vivo microbiological assessment was used as experimental mouse infection model by gram negative strains. Against Pseudomonas aeruginosa F0013, cefoperazone/CH 1240 was slightly less active than sulbactam. ampicillin/CH 2140 was less effective than sulbactam against escheriachia coli 3457. Especially against Citrobacter diversus 2046E, amoxicillin/CH 2140 was the most potent and amoxicillin/CH 1240 was slightly more active than clavulanic acid. consequently the difference in efficacy between the drug combinations appers to be related to the degree of protection afforded the animals by the b-lactamasse inhibitors. CH1240 and CH2140 are promising new agents and should undergo further investigations.