The major heat-inducible protein of transformed human amnion cells (AMA) has been identified as the proliferation-sensitive polypeptide IEF14 (Mr 66 kDa; HeLa protein catalogue). From its mobility in two-dimensional gels (Mr and pI) as well as from the fact that this protein is immunoprecipitated by mAb C92 F3-5 (W. J. Welch, and J. P. Suhan, (1986) J. Cell Biol. 103, 2035-2052), we concluded that this polypeptide is either closely related or identical to the 72 kDa inducible stress human protein hs X 70 (H. R. B. Pelham (1986) Cell 46, 959-961). It is further shown that in AMA cells the rate of synthesis of this protein increases preferentially during mitosis. These results provide further evidence suggesting that the levels of hs X 70 can be modulated by mechanisms independent of heat shock.