This study was attempted to investigate the effect of inosiplex on the cell-mediated immunity in the patients with lepromatous leprosy. Fourteen patients received inosiplex (6gm/day) for 1 month. About 20% of the patients (3 of 14) showed conversion of the lepromin reaction and bacteriological index was significantly decreased in 3 of 7 patients. However, inosiplex administration had no effect on the lymphocyte blastogenesis to M. leprae. The clinical evolution showed a favorable activity of the drug on cutaneous lesions in some patients. The tolerance of the drug was excellent. No side effects were observed. These results suggest that inosiplex may have a moderate immunopotential value in lepromatous leprosy.