Abstract: Thirty cases of LL and BL leprosy were treated for 6 to 12 months with isobutylpiperazinyl rifamycin(R761),Group A:17 cases received R761 150 mg daily for one year,. Group B; 13 cases received 8761 600 mg daily on two successive days every months. plus dapsoue 100 mg daily for six months; Tlie viability of Ml leprae obtained from skin scrapings of seven cases before treatment and 1.2 and 4 weeks after treatment respectively was tested with mouse foot-pad test. The results of clinical trials showed that R761 is effective for dapsone resistant cases, relapsed and new cases of LL and BL leprosy during the period of treatment for 6 to 12 months,Clinical; bacteriological and histopathological improvements in group A were consistent with that in group B in six months of treatment, particularly the skin smears showed that the MI fell rapidly in both groups, but the fall of BI was not as rapid as that treated by other effective anti-leprosy drugs, The.results of mouse foot-pad test showed that M, leprae were killed rapidly, In group A, M, lepraes became non-infective to mice witliin, 8-28 days. and group B within 1-14 days,The severity and frequency of lepra reaction, were.increased-in both groups,Except for two cases with mild hepatic injury no other severe side effects were seen in all cases.