Expts. with 13 strains of Bacillus isolated from canned foods have shown that, like Clostridium spp., the germination of heated spores is affected by inhibitors present in the media. As with Clostridium spp.. susceptibility to inhibitors increases with the amt. of heating to which the spores are exposed. The Bacillus strains, however, show more variation in the extent to which different strains are inhibited in a particular medium. There is also evidence of substantial differences in the amts. or types of inhibitors which are contained in different media. Some of the differences between media are due to variations in the concns. of inhibitors which are adsorbed on charcoal, starch, or serum albumin, but some of the differences are attributable to inhibitors which are not adsorbed on charcoal. Treatment of nutrient agar with charcoal and subsequent removal of the charcoal is as effective in removing inhibitors as incorporation of the charcoal in the medium. The medium, therefore, is the principal or sole source of inhibitors. Attempts to demonstrate inhibitors in the inoculum were unsuccessful. It is unlikely that unsaturated fatty acids account for more than part of the observed inhibition. A suitable adsorbent should be incorporated in media used for evaluating the thermal destruction of Bacillus spores.