The antibiotic activity of cefadroxil was compared with that of cephalexin, cefatrizine, and cefaclor against 472 clinical isolates. Streptococci, staphylcoocci, Enterobacteriaceae, Haemophilus influenzae and Neisseria gonorrhoeae were included in this study. Cefadroxil resembled cephalexin in antibacterial activity. Against most of the bacterial species the activities of the two cephalosporins differed by no more than twofold. Against Streptococcus pyogenes, Str. agalactiae, and the viridans group of streptococci, however, cefadroxil was 3–4 times more active than cephalexin. Cefaclor was generally more active than cefadroxil; the exceptions were Str. pyogenes, the viridans group, Staphylococcus epidermidis, and β-lactamase-producing strains of Staph. aureus, against which neither compound had a measurable advantage. Cefatrizine was more active than cefadroxil against all bacterial species.