The influence of pH of heating menstruum (McIlvaine buffer) on the heat resistance of Bacillus licheniformis was investigated and compared with the heat resistance in homogenised tomato and asparagus at pH 7 and 4 in a wide range of temperatures. Heat resistance was in all mestrua smaller at acid pH. At 99 degrees C and pH 4, heat resistance was 1/20 lower than at pH 7. However, the magnitude of this effect decreased as heat treatment temperatures were increased almost disappearing at 120 degrees C. z values increased from 6.85 at pH 7, to 10.75 at pH 4. At 99 degrees C the effect of pH on heat resistance was constant along the range of pH's tested. The increase of one pH unit increased D99 by 180%. At pH 7 and 4, heat resistance was the same in buffer as in tomato and asparagus homogenates at all temperatures tested. The diminishing influence of the acidification of some foods on the heat resistance of B. licheniformis sterilisation temperatures should be taken into account when a raise in temperature is considered to shorten the duration of heat processes.