The common characteristic of the alpha-crystallin/small heat-shock protein family is the presence of a conserved homologous sequence of 90-100 residues. Apart from the vertebrate lens proteins--alpha A- and alpha B-crystallin--and the ubiquitous group of 15-30-kDa heat-shock proteins, this family also includes two mycobacterial surface antigens and a major egg antigen of Schistosoma mansoni. Multiple small heat-shock proteins are especially present in higher plants, where they can be distinguished in at least two classes of cytoplasmic proteins and a chloroplast-located class. The alpha-crystallins have recently been found in many tissues outside the lens, and alpha B-crystallin, in particular, behaves in many respects like a small heat-shock protein. The homologous sequences constitute the C-terminal halves of the proteins and probably represent a structural domain with a more variable C-terminal extension. These domains must be responsible for the common structural and functional properties of this protein family. Analysis of the phylogenetic tree and comparison of the biological properties of the various proteins in this family suggest the following scenario for its evolution: The primordial role of the small heat-shock protein family must have been to cope with the destabilizing effects of stressful conditions on cellular integrity. The alpha-crystallin-like domain appears to be very stable, which makes it suitable both as a surface antigen in parasitic organisms and as a long-living lens protein in vertebrates. It has recently been demonstrated that, like the other heat-shock proteins, the alpha-crystallins and small heat-shock proteins function as molecular chaperones, preventing undesired protein-protein interactions and assisting in refolding of denatured proteins. Many of the small heat-shock proteins are differentially expressed during normal development, and there is good evidence that they are involved in cytomorphological reorganizations and in degenerative diseases. In conjunction with the stabilizing, thermoprotective role of alpha-crystallins and small heat-shock proteins, they may also be involved in signal transduction. The reversible phosphorylation of these proteins appears to be important in this respect.