Data from longitudinally obtained serum samples spanning several years has permitted us to identify two chimpanzees with leprosy and to estimate the time of Mycobacterium leprae exposure/infection. The results confirm high levels of specific anti-M. leprae phenolic glycolipid-I (PGL-I) as well as anti-lipo-arabinomannan (anti-LAM) antibodies in both chimpanzees, and identify additional chimpanzees with possible M. leprae exposure. The observations are consistent with the hypothesis that leprosy exists in chimpanzees in the U.S.A. and suggest the possibility that M. leprae may be transmitted among chimpanzees. The data suggest that monitoring anti-PGL-I and anti-LAM IgG and IgM levels longitudinally in leprosy contacts may be useful in the recognition of preclinical leprosy.