This study compares the expression after heat shock of the two major variants of the mammalian 70 kilodalton heat shock family in three separate systems. The ability of wild type and temperature sensitive mutant (ts85) FM3A cells to elicit a heat shock response following a 45 degrees C, 12 min exposure was examined. The ts85 cells were found to be both significantly more thermosensitive than parent FM3A cells and to induce a 66kDa heat shock protein (hsp66) not visibly synthesized in the parent line by this exposure. However, a constitutive (synthesized at 37 degrees C) 68kDa heat shock protein (hsp68) is comparably induced in both cell lines after heat. A relationship between the severity of the heat exposure as seen by the cell and hsp66 expression is suggested and tested in Chinese hamster ovary cells. In CHO cells a brief 45 degrees C heat shock induces the constitutive hsp68 (but not hsp66), while longer and more severe exposures are required for the expression of hsp66. The induction of these two proteins is also examined in situ in mouse skeletal muscle. In this case both hsp66 and hsp68 are induced following comparatively mild heat treatments, and the 'threshold' for hsp66 induction observed in cultured cells either does not occur or is greatly reduced. However, once again, hsp68 is naturally synthesized at 37 degrees C while hsp66 appears to be de novo synthesized after heat shock.