The in vitro antibacterial activity of four oral cephems: cephalexin, cefaclor, cefatrizine and cefadroxil against clinical isolates of staphylococci, Enterobacteriaceae and Pseudomonas aeruginosa was compared by agar dilution method. All drugs had comparable antistaphylococcal activity. Cefatrizine and cefaclor were more active than cephalexin and cefadroxil against Escherichia coli, Klebsiella pneumoniae and Proteus mirabilis. All four drugs were relatively inactive against isolates of Enterobacter species and indole-positive Proteus. However, cefatrizine demonstrated the greatest activity of the four oral cephems against these organisms. None of these oral cephems was active against Serratia marcescens or Pseudomonas aeruginosa.