1. The cellular immune response to M. leprae and BCG antigens was evaluated in 98 leprosy patients and 143 household contacts lacking clinical manifestation of the disease. 2. The proliferative responses and release of Interferon-gamma by peripheral blood mononuclear cells were assessed and both patients and contacts were classified as low or high responders to M. leprae. 3. The high responder contacts constituted 54.8% of the population analyzed, a three times higher proportion when compared to the controls, indicating the possible existence of active infection among them. 4. The correlation coefficient between the immunological response to M. leprae and BCG was found to be higher within the contact group than in the patients, suggesting that cross-reactivity defense mechanisms against mycobacteria exist even before the onset of clinically detectable disease.