The minimal inhibitory concentration (MIC) of Ro 13-9904 against 245 clinical isolates was determined by an agar dilution method. The activity of Ro 13-9904 against most Enterobacteriaceae was similar to that of cefotaxime; it was slightly more active than cefotaxime against Proteus mirabilis, Providencia species, and Serratia marcescens, but slightly less active against Klebsiella species. Ro 13-9904 was twofold more active than cefotaxime and threefold more active than ticarcillin against ticarcillin-susceptible Pseudomonas aeruginosa, with a mean MIC of 7.2 micrograms/ml; isolates highly resistant to ticarcillin were inhibited by a mean MIC of 17.2 micrograms/ml. Ro 13-9904 was fourfold more active than ampicillin against susceptible Haemophilus influenzae and was equally active against beta-lactamase-producing isolates. Ro 13-9904 was highly active against pneumococci and moderately active (MIC, 4 micrograms/ml) against Staphylococcus aureus isolates, whether they were susceptible or resistant to penicillin G. Oxacillin-resistant S. aureus and Streptococcus faecalis were completely resistant to Ro 13-9904 (MIC, greater than 128 micrograms/ml).