Hsp20 is one of the newly described members of the mammalian small heat-shock protein (sHsp) family. It occurs most abundantly in skeletal muscle and heart. We isolated clones for Hsp20 from a rat heart cDNA library, and expressed the protein in Escherichia coli to characterize this little known sHsp. Recombinant Hsp20 displayed similar far-ultraviolet circular dichroism spectra as the most closely related sHsp, alpha B-crystallin, but was less heat stable, denaturing upon heating to 50 degrees C. While other mammalian recombinant sHsps form large multimeric complexes, Hsp20 occurs in two complex sizes, 43-kDa dimers and 470-kDa multimers. The ratio between the two forms depends on protein concentration. Moreover, Hsp20 has a much lower chaperone-like activity than alpha B-crystallin, as indicated by its relatively poor capacity to diminish the reduction-induced aggregation of insulin B chains. Hsp20 is considerably shorter at the C-terminus and less polar than other sHsps, but 1H-NMR spectroscopy reveals that the last 10 residues are flexible, as in the other sHsps. Our findings suggest that Hsp20 is a special member of the sHsp family in being less heat stable and tending to form dimers. These properties, together with the shorter and less polar C-terminal extension, may contribute to the less effective chaperone-like activity.