Hydrotropes are a class of compounds that, at high concentrations, enhance the solubility of hydrophobic substances in water. Hydrotropy operates beyond a particular concentration known as the minimal hydrotrope concentration (MHC), above which the hydrotrope molecules begin to self-aggregate and form loose noncovalent molecular assemblies. In this context, we have studied the self-aggregation and hydrotropic behavior of the naturally occurring amino acid L-proline and its derivative 4-hydroxyproline. The solubilizing ability of proline is comparable to that of conventional hydrotropes, while hydroxyproline does not display significant hydrotropic ability, presumably due to the hindrance offered by the hydroxyl group toward self-aggregation. However, the microenvironmental features offered by the proline molecular assembly are found to be somewhat different from those displayed by the conventional hydrotropes and detergents. Proline is known to be a compatible solute in water-stressed cells. Proline delays the thermal unfolding of the protein alpha-chymotrypsin by approximately 10 degrees C, indicating its stabilizing ability on protein structure and conformation and suggesting its ability to act as a "protein-compatible" hydrotrope.