Cefazolin is a semi-synthetic derivative of cephalosporin C that has a lower cross-immunogenicity with penicillins than do the other cephalosporins. This agent was evaluated as an alternative to penicillin in the therapy of patients with pneumococcal pneumonia. Thirty patient were treated with cefazolin, most receiving 125 or 250 mg IM every 12 hours for 5-10 days. Satisfactory clinical responses were obtained in 29 of these 30 patients, and none complained of pain following IM injections. Three patients developed eosinophilia while receiving cefazolin, and one of these also had a maculopapular eruption that may have been an allergic reaction to cefazolin. Serum levels of cefazolin were measured at 1, 6, and 12 hours after administration. Susceptibilities of 100 isolates of Streptococcus pneumoniae, including these patients' organisms, were determined by broth dilution. Both cefazolin and cephalothin were bactericidal for all 100 isolates at concentrations of 2 microgram/ml or less. Cefazolin appears to be an entirely adequate alternative to penicillin for the therapy of pneumococcalpneumonia. This agent is effective in low dosages, and adequate serum levels are maintained for long periods of time, permitting twice-daily administration.