Upon exposure to heat shock, HeLa cells synthesize a small set of proteins having the molecular weights of 70,000, 73,000, 78,000, 85,000, 92,000, and 105,000. In addition to these proteins, we found an unusual heat shock protein induced by heat shock at 42 degrees C, but not at 45 degrees C. The 42 degrees C-specific protein, the molecular weight of which was 90,000, was not produced in control cells and the induction of the protein was completely inhibited by actinomycin D. The protein was not induced by other treatments that induced most heat shock proteins. Thus, this 42 degrees C-specific protein seems to have a peculiar induction mechanism and a specific function in the cells.