Twenty-one previously untreated lepromatous patients were randomized into two groups and treated with either 800 mg pefloxacin (PEFLO) or 400 mg ofloxacin (OFLO) once daily. The trial consisted of two parts: monotherapy from day 0 to day 56; and combined with the World Health Organization multidrug therapy (WHO/MDT) regimen for multibacillary (MB) leprosy from day 57 to day 180. Four patients were removed from the trial because the organisms recovered from their pretreatment biopsies failed to infect mice. Among the remaining 17 cases, four (23.5%) had primary resistance to dapsone but all of them were susceptible to rifampin. The initial (day 0) proportion of viable organisms, as measured by mouse foot pad inoculation, varied tremendously from patient to patient despite randomization during admission. Definite clinical improvement was noticed in virtually all patients after 22 doses of treatment with either PEFLO or OFLO. A significant fall in the morphological index (MI) occurred as early as after 8 doses of PEFLO or after 22 doses of OFLO; the bacterial load also showed a moderate degree of reduction during the period of monotherapy. Although single-dose PEFLO or OFLO displayed only a modest degree of bactericidal effect against Mycobacterium leprae, about 99.9%, or 4 logs, of organisms viable on day 0 were killed by 22 doses of either PEFLO or OFLO. No significant difference in the therapeutic effect was detected between the two regimens. During PEFLO or OFLO monotherapy, except in one patient (case no. 10), the side effects were few and mild. Case no. 10 developed a psychic disorder after 27 days of PEFLO monotherapy, presumably due to the treatment with PEFLO. All of the patients tolerated the period of combined therapy extremely well, although some asymptomatic and transient laboratory abnormalities were observed. Because both PEFLO and OFLO displayed rapid bactericidal activities in human leprosy and were well tolerated by the patients, further clinical trials and field trials in evaluating the therapeutic effects of combined regimens containing both rifampin and PEFLO or OFLO are being organized. Since this is the first clinical trial in leprosy employing nude mice, in combination with normal mice, for monitoring the therapeutic effects of antimicrobials, the advantages, limitations and appropriate timing in using nude mice are discussed.