The nonisothermal transitions of lyophilized recombinant bovine somatotropin (rbSt) as seen via differential scanning calorimetry were evaluated with respect to moisture content. The transition peak temperature of rbSt decreased with increasing moisture from 161 degrees C in the dry state to a plateau of 65 degrees C at 28% moisture, which is similar to that of rbSt in solution. Using high performance liquid chromatography, this irreversible endothermic transition consisted primarily of unfolding, hydrophobic aggregation, and some covalent modifications. In the dry state, covalent modifications, including polymerization into compounds of higher molecular weight, were more prominent, while in the presence of moisture, hydrophobic aggregation was most prominent. The irreversibility and scan rate dependence of the endothermic phenomena supports the kinetic nature of the transition rather than a simple equilibrium between globular and unfolded states. The apparent activation energy for the net transition (i.e., unfolding, hydrophobic aggregation, and covalent modifications) was 57 kcal/mol for rbSt at 9.9% moisture. The observed enthalpy of the transition increased, decreased, then approximately leveled off as a function of increasing moisture content. This can be explained by the increasingly significant contribution of the exothermic aggregation at higher moisture contents.