The present report reviews the significant in vitro characteristics of cephalexin based on data published since 1967 (Wick, 1967). Whenever possible, current data on the action spectrum of cephalexin are compared with previous results, and the significance of in vitro activity to therapeutic efficacy is evaluated in the light of current clinical reports. Recent observations are presented on the in vitro action of cephalexin against newly recognized bacterial pathogens, such as beta-lactamase-producing Haemophilus influenzae and Branhamella catarrhalis (formerly Neisseria catarrhalis). Data are presented on the stability of cephalexin to various beta-lactamases. Current information on the types of penicillin-binding proteins that interact with cephalexin is briefly noted. Finally, consideration is given to the effects of inoculum density on the minimum inhibitory and bactericidal concentrations of cephalexin. The continued utility of the 30 micrograms cephalothin-class disc to predict cephalexin susceptibility is emphasized.