alpha-Crystallin, the major protein of the ocular lens, is known to have extensive similarities to small heat shock proteins and to act as a molecular chaperone. The exposure of hydrophobic surfaces on alpha-crystallin was studied by fluorescence spectroscopy using the hydrophobic probe bis-ANS. Upon heating the protein undergoes a conformational transition which is associated with a marked increase in surface hydrophobicity. This transition, which occurs between approximately 38 and 50 degrees C, lacks reversibility. The increase in surface hydrophobicity correlates with the increased chaperone activity of the protein. These results indicate that hydrophobic interactions play a major role in the chaperone action of alpha-crystallin.