Ciprofloxacin activity against Leptospira interrogans serogroup icterohaemorrhagiae was studied in vitro and in an animal model. The MBC of ciprofloxacin was 0.6 microgram/ml. Three of three Syrian hamsters died 8 to 9 days after intraperitoneal challenge with 10(6) leptospires. In contrast, five of six animals given ciprofloxacin 3 or 5 days after challenge survived.