The addition of isopropyl thio-beta-D-galactoside (IPTG) to Escherichia coli cells containing multiple copies of the heat shock regulatory gene htpR (rpoH) under the control of an IPTG-inducible promoter (P-tac) induced 15 of the 17 polypeptides of the heat shock (HTP) regulon. The time course and magnitude of the induction closely resembled that caused by a shift to 42 degrees C. Nevertheless the two means of inducing the heat shock regulon differed in outcome. Cultures grown at 28 degrees C and induced by incubation at 42 degrees C for 15 min gave significant protection against a challenge temperature of 50 degrees C, but no protection was afforded by a 15-min IPTG treatment at 28 degrees C. It could be shown that there was no interference by IPTG with the development of thermotolerance at 42 degrees C. Also, treatment of a wild strain of E. coli with various toxic agents revealed no correlation between the development of thermotolerance and the induction of any subset of the heat shock proteins. Thermotolerance appears to develop by processes other than the htpR-dependent induction of heat shock proteins.