Refrigerated processed foods of extended durability rely on a mild heat treatment combined with refrigerated storage to ensure microbiological safety and quality. The principal microbiological safety risk in foods of this type is non-proteolytic Clostridium botulinum. In this article the combined effect of mild heat treatment and refrigerated storage on the time to growth and probability of growth from spores of non-proteolytic Cl. botulinum is described. Spores of non-proteolytic Cl. botulinum (two strains each of type B, E and F) were heated at 90 degrees C for between 0 and 60 min and subsequently incubated at 5 degrees, 10 degrees or 30 degrees C in PYGS broth in the presence or absence of lysozyme. The number of spores that resulted in turbidity depended on the combination of heat treatment, incubation time and incubation temperature they received. Heating at 90 degrees C for 1 or more min ensured a 10(6) reduction when spores were subsequently incubated at 5 degrees C for up to 23 weeks. Heating at 90 degrees C for 60 min ensured a 10(6) reduction over 23 weeks when subsequent incubation was at 10 degrees C in the presence of added lysozyme. The same treatment did not reduce the spore population by 10(6) when subsequent incubation was at 30 degrees C.