Differential scanning calorimetry was used to study the thermal unfolding of hevein, a 43-residue disulfide-rich protein whose three-dimensional structure has been determined by X-ray diffraction. In the range pH 2.0-3.7 this process was approximately 75% reversible as judged by repeated scans on the same sample. The ratios of van'tr Hoff to calorimetric enthalpies were considerably larger than one, suggesting that intermolecular cooperation is involved in the unfolding of this protein. Alternatively, it is possible that the partial irreversibility of this process may cause distortions of the endotherm that affect the calculation of the van't Hoff enthalpy. Experimental changes in heat capacity and enthalpy were compared with those calculated from polar and nonpolar surface areas buried in the native state. It was found that when the unfolded state is represented as an extended chain without disulfide cross-links, experimental and calculated parameters agree well. However, if the unfolded protein is modeled with the presence of disulfide bridges, the agreement between the two sets of parameters is lost. The entropy change/residue at 112 degrees C is considerably smaller than the average value for globular proteins, thus suggesting that, as expected, disulfide bonds strongly influence the entropy of the unfolded state of this protein.