Heat killed tubercle bacilli repeatedly injected into or below the skin of rabbits increase conspicuously their resistance against infection with virulent tubercle bacilli. Protection against tuberculous infection following the administration of heat killed tubercle bacilli to rabbits is only slightly less than that given by BCG. Addition of certain antigens, notably heated horse serum, increases the protection given by heat killed tubercle bacilli so that it is approximately the same as that afforded by BCG. These experiments and tentative observations of persons exposed to tuberculous infection indicate that heat killed tubercle bacilli may be substituted for the living attenuated microorganism in the attempt to increase resistance against tuberculous infection and to influence favorably the delicate balance between asymptomatic or latent infection and progressive manifest disease that is characteristic of human tuberculosis.