Efficacy of ciprofloxacin and pefloxacin in plague infection was studies on albino mice by comparison with that of nalidixic acid. The mice were contaminated subcutaneously. The treatment with the drugs was started 6 or 24 hours after the contamination. The MICs of ciprofloxacin, pefloxacin and nalidixic acid were 0.01 to 0.02, 0.15 and 1.25 to 2.5 micrograms/ml respectively. By the ED50 ciprofloxacin proved to be the most efficient. Pefloxacin was 6 times less efficient and nalidixic acid was 100 times less efficient. The quinolones were shown to be highly efficient when administered at 48-hour intervals.