The in vitro antibacterial activity of sulbenicillin against a number of mucoid and non-mucoid strains of Pseudomonas aeruginosa was investigated and compared with that of some other beta-lactam antibiotics. From the data reported it is evident that sulbenicillin showed better anti-microbial activity than carbenicillin in almost all the tests run. Sulbenicillin appears to have a somewhat lower activity than piperacillin and cefotaxime; however, cefotaxime and particularly piperacillin are highly conditioned by the inoculum size and have a less favourable MBC to MIC ratio.