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Spores of Bacillus stearothermophilus were exposed to calcium and sodium salts of dipicolinic acid (DPA) in phosphate and Tris acid maleate buffers over the range pH 4.5–10.0. The exposed spores were enumerated using a standard plate counting technique from which the kinetics of colony formation were determined and maximum colony counts were obtained for each condition examined. Exposure of the spores to calcium‐DPA (50‐40 mmol/l) in Tris acid maleate buffer pH 9.0 maintained at 10°C was found to produce an optimal response. Following this method the total viable population of a spore suspension was enumerated. This was demonstrated statistically using the Wilcoxon rank‐sum test for significance. Calcium‐DPA was found to produce activation in spores but further germinants and nutrients were required for colony formation. The Ca‐DPA treatment was found to be effective in enumerating both naturally dormant spores and heat injured spores.

Instability and non‐uniformity of spore preparations and the non‐conformity of chemical indicators to the temperature coefficients of spore inactivation are problems associated with current methods of autoclave cycle monitoring. A prototype micro‐electronic instrument, which largely overcomes these problems, is described. It monitors autoclave cycles in terms of the integral F0 and Nabla functions. Its thermometric and integrating accuracy is demonstrated. A discussion of the problems associated with the use of sensitive electronic instruments in the autoclave room environment reveals the need for independent monitoring areas when such devices are used. Inactivation constants were determined for spores of the organism Bacillus stearothermophilus, NCTC 10 003 and were described using D115 (12.0 min) and Z (9.0 °C) values and the first order reaction Arrhenius constants A (1041, 2 min−1) and Ea (74.4 kcal mol−1, 311 kJ mol−1). These have been compared with recently published values. The standardization of F0 and Nabla is discussed with reference to the setting of minimum values related to product bioburden and maximum values related to an acceptable degree of product degradation.