The level of antimicrobial activity of 3-desacety1cefotaxime was found to be approximately eightfold lower than that of cefotaxime. Variation in the inhibitory effects of desacety1cefotaxime was species dependent. Desacetylcefotaxime was 1–2 log2 dilutions more active than cefotaxime against Pseudomonas acidovorans and Pseudomonas cepacia. Desacety1cefotaxime and cefotaxime were bactericidal, and their minimal inhibitory concentrations were influenced only slightly by increases in the inoculum concentration. Synergy (complete and partial) of 75.8% was noted with the combination. Only with strains of Morganella morganii was the combination antagonistic, but these results were not clinically significant; i.e., all strains remained susceptible to cefotaxime.