Cephaloridine was used to treat 136 bacterial infections in 122 hospitalized patients. Its antibacterial and clinical effectiveness against serious staphylococcal infections compared favorably with that of cephalothin sodium and of penicillinase-resistant penicillins in similar cases. Cephaloridine was highly effective clinically and in eradicating the causative organisms in infections due to pneumococci, streptococci, Escherichia coli, and Proteus mirabilis; but its effectiveness against klebsielleae was variable. Superinfection with resistant gram-negative bacilli and serious noninfectious conditions were the major causes of most of the deaths. Injections of cephaloridine were remarkably well tolerated. Cross sensitization with penicillin was not encountered. Cephaloridine may have caused or contributed to acute renal failure and to an unusual coagulopathy, each observed in two patients. Cephaloridine is a highly effective and useful antibacterial agent when selected and used with due regard to its activity and potential nephrotoxicity.