Serum complement profile studied in 50 lepromatous leprosy patients with various bacillary loads demonstrated significantly decreased C3 levels in patients with high bacteriological index (2+ to 4 +) as compared with those with lesser bacterial load. In contrast, mean serum levels of Cl q and C4 components remained unchanged. The concentration of factor B breakdown product (Ba) and its ratio to factor B increased with the bacterial density and more so in patients with erythema nodosum leprosum. A significant negative correlation was found between serum C3 and Ba levels in most lepromatous patients. Analysis of the data suggested the alternative pathway as the possible mechanism of complement activation in lepromatous leprosy.