Human heat shock protein of apparent molecular mass 20 kDa (Hsp20) and its mutant, S16D, mimicking phos-phorylation by cyclic nucleotide-dependent protein kinases, were cloned and expressed inEscherichia coli. The proteins were obtained in a homogeneous state without utilization of urea or detergents. On size exclusion chromatography at neutral pH, Hsp20 and its S16D mutant were eluted as symmetrical peaks with an apparent molecular mass of 55–60 kDa.