The protein alpha-crystallin, a member of the small heat shock protein family, has the ability to prevent aggregation of partially denatured proteins, an effect demonstrated both in vivo and in vitro. In this work, we have probed the apparent thermal destabilization of bovine insulin by alpha-crystallin, using differential scanning calorimetry, near-UV circular dichroism, and intrinsic fluorescence spectroscopy. The thermal denaturation of insulin, followed by differential scanning calorimetry, is greatly affected by the presence of alpha-crystallin. Even at a ratio of alpha-crystallin subunit to insulin monomers as low as 1:10, a significant decrease in the transition temperature and a change in the shape of the transition are evident. These changes are detected for both zinc-free (mainly dimeric) and zinc-containing (predominantly hexameric) insulin. The transition temperatures measured by near-UV circular dichroism are consistent with the calorimetry results; however, no changes in the spectra of insulin occur below the transition temperature in the presence of alpha-crystallin. The intrinsic fluorescence of alpha-crystallin indicates association with insulin above 40 degrees C. On the basis of this, we conclude that alpha-crystallin promotes the dissociation of insulin oligomers to a lower-association state species with a lower thermal stability. Furthermore, we propose that the dissociation of insulin is caused by the ability of alpha-crystallin to bind to the insulin self-association surfaces and thus stabilize insulin dimers and monomers.