Immunotherapy with a vaccine consisting of autoclaved Mycobacterium w was given in addition to chemotherapy in 54 multibacillary, lepromin negative patients belonging to BB, BL and LL types of leprosy. Thirty-seven patients with similar types of diseases received chemotherapy and placebo injections. The 'vaccine' was repeated every 3 months. Bacterial clearance was more rapid in the vaccinated patients. Two lepromatous leprosy patients with initial bacterial index (BI) of 1.8 and 2.8 became bacteriologically negative in 1 year. One LL patient with BI of 6.0 had a BI fall to 0.16 after four doses of the vaccine. None of the LL patients belonging to placebo group during the same time period became bacteriologically negative. Rapid bacterial clearance was accompanied by distinct signs of clinical improvement. One hundred percent of BB, 85.7% of BL patients and 61.5% of LL patients converted to lepromin positivity after four doses of the vaccine. A significant number of vaccinated patients demonstrated an upgrading in skin lesions histopathologically.