In vitro antibacterial activities of a new cephalosporin, ceftriaxone, were bacteriologically characterized in comparison with those of cefotaxime and ceftizoxime. Minimal inhibitory concentrations (MIC) of ceftriaxone determined on 680 fresh clinical isolates in Japan showed extraordinarily high activity against all gram-negative bacteria. Especially notable was its high activity against Proteus spp. and Haemophilus influenzae; in this respect it was greatly superior to cefotaxime and ceftizoxime. It also showed high activity against Pseudomonas aeruginosa and some anaerobic pathogens. Against other strains, in general, ceftriaxone exhibited activity comparable to those of 2 structurally related cephalosporins, except for Klebsiella sp. and P. maltophilia, against which it showed lower activity. Its activity is bactericidal and, in contrast to cefotaxime and ceftizoxime, its minimal bactericidal concentration (MBC) value was < 3 times the MIC except for P. aeruginosa. Its mode of action was morphologically assessed. Ceftriaxone showed an unusually high stability to most bacterial .beta.-lactamases except to cefuroximases from Bacteroides fragilis, P. cepacia and P. vulgaris. Ceftriaxone is a very potent inhibitor of cephaloridine hydrolysis by various .beta.-lactamases.