Susceptibilities of 259 isolates of pathogenic bacteria to cefazolin were measured by broth and agar dilution procedures. Beta-hemolytic streptococci were inhibited by 0.25 μg/ml, whereas Staphylococcus aureus and alphahemolytic streptococci were inhibited by 2.0 μg/ml. Enterococci were resistant to less than 32 μg/ml. Wide variation was seen with gram-negative species. Most isolates of Klebsiella species and Proteus mirabilis were inhibited by 4.0 or 8.0 μg/ml. Escherichia coli were less susceptible, and most isolates of Pseudomonas aeruginosa, Serratia species, and Enterobacter species were resistant to 128 μg/ml.