The htpR locus in Escherichia coli encodes a regulator of the heat shock response. Cells containing the htpR165 mutation are defective in the induction of synthesis of heat-shock proteins at high temperature. We show that these cells are also defective in degrading two proteins that are normally unstable in htpR+ cells. The proteolytic defect is manifest at both 30 degrees C and 42 degrees C. We used a marker rescue technique to map this defect to the htpR locus. Although both proteolytic substrates are partially stabilized in lon- strains, we argue that the defect in proteolysis exhibited by the htpR165 strain does not mimic the lon- state. The htpR165 strain synthesizes Lon at the normal rate at 30 degrees C and does not show the phenotypes of mucoidy and radiation sensitivity associated with lon- strains.