Escherichia coli O157:H7 has been reported as being not particularly heat resistant. However, several factors which might increase its heat resistance have been investigated in this study using five strains. Increase in growth temperature to 40 degrees C, as found in the cow gut, heat-shock at sub-lethal temperatures of 42, 45, 48 and 50 degrees C, and variable heating rate (1 degree C min-1 to 23 degrees C min-1) had no dramatic effect on heat resistance. Growth phase had a marked impact on heat resistance; late stationary phase cells were more heat-resistant than were log phase cells. The difference in heat resistance between the two phases of growth became more pronounced when cells were resuspended in fresh nutrient broth; heat resistance of late stationary phase cells increased dramatically whereas no such effect was observed with log phase cells. The addition of polyphosphates to the heating medium did not increase heat resistance. A reduction in water activity of the heating medium from 0.995 to levels between 0.980 and 0.960 also resulted in a marked increase in heat resistance. This effect was more pronounced under conditions of extremely low water activity created by resuspending late stationary phase cells in sunflower oil. Survivors were detected even after a heat treatment at 60 degrees C for 1 h or 70 degrees C for 5 min. It can be confirmed that this serotype has no unusual heat resistance and that the heating environment markedly affects resistance.