Thermotolerance and heat resistance are frequently associated with elevated levels of heat shock proteins (HSPs). Elevated heat resistance is also found to be associated with the overexpression of high levels of HSP70, as seen in M21 cells, derived from the Rat-1 line. In the present study, we report that M21 cells also feature an increase in general protein glycosylation and specific expression of the stress glycoprotein, GP62, both of which correlate with cellular heat resistance. The expression of GP50, a major stress glycoprotein in cell lines such as CHO, however, did not correlate with cellular heat resistance in M21 cells. Protein glycosylation that occurs during acute heat stress ("prompt" glycosylation) was associated with the glycosylation of a major "prompt" stress glycoprotein, P-SG64 (M(r) of 64,000), that was identified by immunoblotting as a glycosylated form of calreticulin. The higher level of protein glycosylation in M21 cells correlated well with increased D-[2-3H]mannose incorporation into precursor pools of dolichyl phosphomannose and dolichyl pyrophosphoryl oligosaccharides and into glycoproteins. Thus, heat resistance in M21 cells is associated not only with expression of high levels of HSP70, but also with a concomitant increase in protein glycosylation. These data support the hypothesis that stress-induced protein glycosylation is a component of cellular stress response, either in association with HSPs or as an independent mechanism.