A partially purified lipid fraction from Mycobacterium leprae yielded distinct lines of precipitation with antisera from two lepromatus patients and from an infected armadillo. There was no reaction to the sera from patients with tuberculosis or a M. avium infection or to the serum from a normal armadillo. The activity in the lipid fraction was unaffected by mild alkali, and upon hydrolysis the fraction yielded 6-deoxyhexoses. This information suggests that the lipid antigens of M. leprae may be species specific and related to the A, B, C or G mycosides.