The in vitro activities of 16 antibiotics against five serovar strains of the genus Leptospira were determined. Five of the antibiotics (ampicillin, cefmetazole, moxalactam, ceftizoxime, and cefotaxime) exhibited a lower minimal inhibitory concentration than did penicillin G. In tests for minimal bactericidal concentration, ceftizoxime and cefotaxime were found to be more effective than penicillin G, streptomycin, tetracycline, ampicillin, and cefmetazole.