Seven patients with bacterial endocarditis and two patients with bacteremia were treated with cefazolin, with excellent bacteriologic and good clinical response. Dosage of cefazolin was 4-12 g daily. Large doses of cefazolin (12 g/day) were tolerated well by these patients without evidence of hematotoxicity, hepatotoxicity, or nephrotoxicity. Cefazolin is an effective antimicrobial agent for treatment of serious infections caused by susceptible gram-positive organisms. Cefazolin is a derivative of 7-aminocephalosporanic acid, which is bactericidal for Staphylococcus, group A Streptococcus, Diplococcus pneumoniae, several members of the order Enterobacteriaceae, particularly E. coli and Klebsiella pneumoniae, and certain Shigella and Salmonella species [1-4]. Cefazolin may be administered iv or im. It appears to be less painful than cephalothin when administered im, and preliminary results suggest less nephrotoxicity than that reported for cephaloridine [5-11]. The present study is a report of cefazolin therapy in seven patients with bacterial endocarditis and two with pneumococcal bacteremia.