Immune responsiveness toMycobacterium lepraewas studied in various groups of healthy humans. Contacts of leprosy patients responded significanctly more than non‐contacts by the methods of leucocyte migration inhibition, lymphocyte transformation and early and late lepromin testing. By classifying responses of strengths found in non‐contacts as negative, 71.2 per cent of medical attendants, the main category of contacts, were responders by the leucocyte migration inhibition test, 44.2 per cent by the lymphocyte transformation assay and 50.0 per cent by the early lepromin reaction. On the other hand, no degree ot the late lepromin reaction was found solely inM. leprae‐exposed people. While the assays of leucocyte migration inhibition, lymphocyte transformation and early lepromin testing thus may be considered useful for detection of healthy individuals exposed toM. leprae, the late lepromin reaction appears unsuitable as a measure of exposure. Besides the association of negative responses by leucocyte migration inhibition, lymphocyte transformation and early lepromin tests, there was in the group of non‐contacts a significant quantitative correlation between early and late lepromin reactions. In the group of medical attendants significant correlations were observed between the results of all tests e