Proline in aqueous solution shows several properties which are unusual for low molecular weight substances. Investigations of solubility, density and viscosity revealed behaviour which is characteristic for hydrophilic colloids. 1H-NMR studies indicated a strong hydrogen bonding of water in proline solutions, especially at high concentrations of the solute. From these results it was concluded that proline forms aggregates by stepwise stacking and hydrophobic interaction of the pyrrolidine ring. Thus, the proposed multimer contans a hydrophobic backbone and hydrophilic groups on the surface, exposed to water. Proline solutions are able to increase the solubility of sparingly soluble proteins. The enhancement effect depends on the nature of the protein and on the proline concentration. It is postulated that by a hydrophobic interaction of proline with hydrophobic surface residues of proteins their hydrophilic area is increased. The presence of proline in solutions of the well soluble protein bovine albumin reduces the precipitation of this protein by ethanol and (NH4)2SO4, presumably by an increased water-binding capacity of the proline-protein solution.