SummaryExperiments with 13 strains of types A and B Cl. botulinum have shown that the detection and enumeration of spores surviving lethal heating become increasingly difficult as the period of heating is increased. Media which are very satisfactory for enumerating unheated spores may be unsatisfactory for enumerating spores which survive the heat treatment. By comparison with unheated spores the numerical estimates of spores surviving severe heat treatments were more dependent on thioglycollate in the medium and on the pH and temperature during incubation. As the amount of heating was increased, the surviving spores became progressively more sensitive to inhibitors· in the medium. These inhibitory substances were present in all media used, and their effects were counteracted under a wide range of conditions by the incorporation of approximately 0.1 per cent. starch in the medium. The starch supplement produced no significant effect on the estimates of unheated spores. The effects of starch are a feature of the amylose portion of the undegraded starch molecule which, like charcoal or serum albumin, acts by adsorbing inhibitors from the medium. The probability that at least part of the inhibition is due to unsaturated fatty acids is discussed.The heated spores of several other species of Clostridium show a similar response to the inclusion of starch in the medium.