A single 1-Gm dose of cefazaflur, a new semisynthetic cephalosporin derivative, was compared in a crossover study to the same dose of cephalothin and cefazolin by intramuscular injection in seven healthy volunteers. Serum concentrations were measured at several time intervals during 6 hours following each administration. The mean peak serum levels obtained after 30 minutes were 25.2, 17.2, and 62.3 mug/ml, respectively, for cefazaflur, cephalothin, and cefazolin. In each of the seven subjects, serum concentrations were higher at each sampling time with cefazolin than with the other two cephalosporins. The percentage of total administered dose recovered in urine in a microbiologically active form for the 0-24-hour collection was, respectively, 92.7, 59.2, and 94.9 per cent with cefazaflur, cephalothin, and cefazolin, the largest part being excreted during the first 6 hours. Neither drug appeared to have any pronounced effect on various laboratory tests. Local reactions at the site of intramuscular injection were minor with cefazaflur and cefazolin, but were more pronounced with cephalothin.