Spores of Bacillus subtilis MD2 and Bacillus subtilis var. niger were heat activated for different times at 60° and 80°C. Strain MD2 required considerable heat activation while B. subtilis var. niger did not. Maximum germination rates increased with heat activation dose and declined subsequently without loss of germinability. Germination rates and percentages were considerably greater in tryptone glucose extract (TGE) than in nutrient broth. The addition of 2°° dimethyl sulphoxide did not increase germination in nutrient broth. The spores of var. niger are more resistant to dry‐heat than MD2 although they are less resistant to moist heat. Survivor curves in the dry‐heat range 140°‐170°C gave D‐values from 4–123 to 0.106 min for MD2 and 5.679 to 0.233 min for var. niger recovered on TGE agar. D‐values were lower on poorer media. The z‐values for MD2 and var. niger on TGE were 18.7°C and 21.25d̀C respectively.

Spores of Bacillussubtilis MD2 and var. niger were dry-heat damaged at 150', 160" and 170°C and recovered on media of increasing complexity. The greater the heat dose the more marked was the effect of amino acid supplements on recovery. For strain MD2 maximum germination and outgrowth of unheated spores could be obtained on a minimal salts + glucose medium with alanine, aspartic acid, glycine and methionine; the latter three amino acids served to enhance growth, not germination. The recovery of heat-damaged spores was significantly increased by adding valine plus isoleucine or arginine or glutamine. The increase was probably due to the use of valine and isoleucine as substrates of NAD-linked dehydrogenases to generate reducing power and serve as NH,-donor, initiating germination in spores which were unable to germinate as a result of inactivation of alanine dehydrogenase. Valine or isoleucine added singly suppressed recovery by feedback inhibition of the pathways to both these amino acids during outgrowth.