The in vitro activity of GR-20263, a new aminothiazolyl cephalosporin, was compared with the activities of other beta-lactam antibiotics by using 800 clinical bacterial isolates. GR-20263 was highly active (inhibition of 90% of the isolates between 0.03 and 1 microgram/ml) against the common Enterobacteriaceae and 5 to 20 times more active than cefuroxime, cefoxitin, and cephalothin. GR-20263 was three to six times less active than cefotaxime against Escherichia coli, Klebsiella pneumoniae, Salmonella, and Shigella, but three to four times more active than cefotaxime against Proteus vulgaris and Serratia marcescens. The activity of GR-20263 against Pseudomonas aeruginosa (with minimal inhibitory concentrations of 2 and 8 micrograms/ml for 90 and 100% of the isolates, respectively) was similar to that of tobramycin, 2 times that of cefsulodin, 5 times that of piperacillin, and 10 times that of cefotaxime. Against Haemophilus influenzae GR-20263 was three time more active than ampicillin. The beta-lactamase-producing strains were as susceptible to GR-20263 as the beta-lactamase-negative strains. GR-20263 was less active than cefotaxime and ampicillin against Staphylococcus aureus.