Cefatrizine (BL-S640), a semisynthetic, orally administered cephalosporin, was found to have an in vitro spectrum of activity comparable to those of four other cephalosporins tested. It is as effective as cephalexin, the other orally administered cephalosporin evaluated, against most species, and it appears to be more effective than cephalexin against many Enterobacter, Haemophilus , and Proteus strains isolated in our hospital. It is not inactivated by the plasmid-determined β-lactamases of 14 strains of ampicillin-resistant Salmonella typhimurium or the ampicillin resistance determinant of an H. influenza strain from the Center for Disease Control. No synergy was observed between cefatrizine and gentamicin, kanamycin, carbenicillin, or polymyxin when tested against selected strains.