The protective effects of ciprofloxacin and rufloxacin were compared to those of rifampicin against Mycobacterium tuberculosis infections in mice and in guinea pigs. Rifampicin was very protective in both models. In tubercular infections produced in mice, ciprofloxacin (30 mg/kg) showed slight protection but none was observed in guinea pigs. Rufloxacin, was weakly active in guinea pigs but inactive in mice.