The in vitro activity of ceftizoxime, a new third-generation cephalosporin, was tested by determining minimal inhibitory concentrations (MIC) by agar dilution method, for 150 strains of genus Pasteurella and group EF4 bacteria from various sources. All the strains were susceptible to ceftizoxime and inhibited by 0.03 micrograms/ml concentration. No significant difference appeared between the 7 species and human and animal strains. Group EF4 bacteria, frequently isolated from animal bite wounds in humans, had higher MIC (O.25-1 micrograms/ml). Results were compared to those obtained with cefotaxime, cephalothin, penicillin G and ampicillin.