Cephanone, a new 3-heterocyclic-thiomethyl cephalosporin antibiotic, was found to have an antibacterial spectrum similar to that of cephalothin. The compound was active in vitro against a variety of gram-positive and gram-negative bacteria. All strains of Staphylococcus aureus tested were inhibited by concentrations of 6.2 mug or less of cephanone per ml. Beta-hemolytic group A streptococci and pneumococci were exquisitely sensitive. Among strains of Escherichia coli and Klebsiella sp., 83 and 82%, respectively, were inhibited by 3.1 mug or less of cephanone per ml. Excellent serum concentrations of the antibiotic were obtained after parenteral administration. Peak concentrations of 38 and 81.2 mug/ml were achieved in the serum after intramuscular and intravenous doses of 1 g of cephanone, respectively. The serum concentrations of cephanone fell gradually during the 12 hr after administration. Very high concentrations of cephanone were found in the urine.