Relatively mild heating of the spores of mesophilic aerobes has been shown to hasten their subsequent germination (Evans and Curran, 1943). Although effective in bringing about a more rapid germination, with one exception, such heating had no measurable influence upon the number of spores that germinated. This tangible evidence of pregermination stimulation led us to extend our observations to a group of thermotolerant aerobes isolated from commercially canned evaporated milk that had spoiled. With nearly all the latter types, preheating was found to exert a determining influence upon the number of spores that germinated. This phenomenon, which must be regarded as true heat activation, has not been reported previously for bacterial forms, although evidence of its operation may be found in the publications of Mudge and Thorwaldsen (1930) and Christian (1931), both of which dealt with obscure milk defects in which heat affected the development of sporeforming organisms. Seeking to explain observed qualitative and quantitative changes in the flora of pasteurized milk, Mudge and Thorwaldsen formulated an interesting hypothesis which assumes for certain thermophiles a complex life cycle involving both visible and invisible spore forms. The latter, normally dormant, might be induced to germinate by physical and chemical agencies, including heat. Christian believed that heating promoted the development of a "coconut" or "carbolic" taint in commercial sterilized milk by destroying a product of vegetative activity inhibitory to the germination of the spores. Some light is shed upon the heat activation reaction by experiments reported in this paper. The relationship between the amount of pregermination heat and the degree of activation, and certain factors exclusive of pregermination heat which affect the heat response are given especial consideration.