Ciprofloxacin at a concentration of 2 µg/mL inhibited the growth of ∼90% of 584 strains of aerobic bacteria isolated from cultures of blood drawn from septicemic patients. An increase in the inoculum size did not result in an increased MIC, but serial passages through media containing ciprofloxacin at sub-MIC levels increased the MIC for Escherichia coli, Klebsiella pneumoniae, and Proteus vulgaris. In experimental subcutaneous abscesses in the mouse model, ciprofloxacin was more active than cefotaxime against a mixed infection induced with E. coli and Bacteroides fragilis. Against mixed E. coli and Staphylococcus aureus infection, no significant differences were noted between the two drugs. In a double-blind, prospective, randomized clinical study, perorally administered ciprofloxacin was compared with intravenously administered cefotaxime in the treatment of skin and soft-tissue infections severe enough to require hospitalization. In 70 patients treated, the therapeutic efficacy of peroral ciprofloxacin was comparable to that of intravenous cefotaxime, with two differences: S. aureus infections responded less favorably to oral ciprofloxacin (62%) than to intravenous cefotaxime (90%), and aerobic gram-negative bacillary infections responded more favorably to ciprofloxacin (92%) than to cefotaxime (64%).