Cefazaflur was tested in vitro against 262 strains of bacteria. When tested in agar, 5.0 mug or less of cefazaflur per ml inhibited almost all strains of Staphylococcus aureus, Escherichia coli, Klebsiella, and Proteus mirabilis. The drug was less active against Enterobacter and indole-positive Proteus, and 7.5 mug of antibiotic per ml inhibited approximately two-thirds to one-fourth of the strains. A concentration of 50 mug of cefazaflur per ml was required for inhibition of the enterococci. There was negligible activity against Pseudomonas. The drug demonstrated less activity in broth than in agar, and a major inoculum effect was seen with some strains. For example, with a lower inoculum, 2.5 mug of cefazaflur per ml killed all strains of E. coli, whereas with the higher inoculum, 7.5 mug of cefazaflur per ml, inhibited 64% and killed only 8% of strains.[1]