T cell proliferation and interferon (IFN)-gamma secretion were analyzed in 45 leprosy contacts stimulated with antigen 85 (Ag85), the major culture filtrate antigen from Mycobacterium bovis bacille Calmette-Guérin. All 14 Mitsuda reaction-positive contacts reacted to Mycobacterium leprae and Ag85. Three Mitsuda reaction-negative contacts reacted weakly to M. leprae and Ag85. The other 28 Mitsuda reaction-negative contacts did not react to M. leprae, but 9 reacted to Ag85. Thirty-four contacts were retested 16 months later. Eleven contacts initially positive by the Mitsuda test remained lepromin positive and reactive to M. leprae and Ag85. Fourteen contacts initially negative by the Mitsuda test converted, and all reacted in vitro to M. leprae and Ag85. Finally, 9 contacts remained Mitsuda test-negative, and 7 were unreactive to Ag85. In vitro reactivity to Ag85 at baseline in Mitsuda test-negative contacts was associated with subsequent conversion to lepromin reactivity in 7 of 9 subjects. These data suggest that reactive T cells against Ag85 develop very early during M. leprae infection and that Ag85 is a potentially protective T cell immunogen.