The Streptococcus milleri group are uniformly susceptible to penicillin G, but their susceptibilities to different cephalosporins vary considerably. The antimicrobial susceptibilities of 100 clinically significant strains of the S. milleri group to 16 cephalosporins were determined by the agar dilution method. The majority of first-generation cephalosporins were highly active. Cefamandole, cefuroxime and cefprozil were the most active second-generation agents examined. Third-generation parenteral cephalosporins exhibited excellent activity, with the exception of ceftazidime. The most active of the oral preparations of this group was cefpodoxime, with cefixime and ceftibuten being considerably less active. MICs of cefepime, the only fourth-generation cephalosporin tested, were higher than those of cefotaxime and ceftriaxone.