Heat resistance of Lactobacillus bulgaricus in skimmed milk at 62 degrees, 64 degrees, 65 degrees and 66 degrees C was studied. The response to increasing temperatures in this range was not linear, with temperatures at 65 degrees C and above giving a lower survival rate than would be predicted from experiments at lower temperatures. To identify sites of injury at these temperatures, chemical markers were used. Heating at 64 degrees C and below resulted in damage to the cytoplasmic membrane. At temperatures of 65 degrees C and above chemical markers also indicated damage in the cell wall and proteins. Using differential scanning calorimetry analysis of whole cells of Lact. bulgaricus seven main peaks were observed (1-51, m1-61, m2-73, n-80, p-89, q-100, r-112 degrees C). Three of these peaks (l(r), m(r) and p(r)) were the result of reversible reactions. Analysis of cell fractions identified the cell structure involved in giving rise to each of the three reversible peaks; l(r), cell membrane lipids, m(r), ribosomes, and p(r), DNA. The evidence presented in this paper shows that irreversible reactions in the cell ribosomes are a critical site of damage in Lact. bulgaricus during heat stress in liquid media at 65 degrees C and above.