The thermal denaturation of bovine serum albumin (BSA) was studied at pH 2.8 and 7.0 in the range of 2-65 degrees C. The relative proportions of alpha-helix, beta-structure, and disordered structure in the protein conformation were determined as a function of temperature, by the curve-fitting method of circular dichroism spectra. With the rise of temperature at pH 7.0, the proportion of alpha-helix decreased above 30 degrees C and those of beta-structure and disordered structure increased in the same temperature range. The structural change was reversible in the temperature range below 45 degrees C. However, the structural change was partially reversible upon cooling to room temperature subsequent to heating at 65 degrees C. On the other hand, the structural change of BSA at pH 2.8 was completely reversible in the temperature range of 2-65 degrees C, probably because the interactions between domains and between subdomains might disappear due to the acid expansion. The secondary structure of disulfide bridges-cleaved BSA remained unchanged during the heat treatment up to 65 degrees C at pH 2.8 and 7.0.