Rat 13762NF mammary adenocarcinoma cells (clone MTC) were heated to 42°C either in vivo as a subcutaneous tumour in the rat mammary fat pad or in vitro as attached cells. Labelling in vivo or in vitro detected very similar heat-stress proteins (hsp) at 160, 112, 90, 70 and 56kDa. Syngeneic rat endothelial and macrophage cells synthesized several cellular proteins in vitro differently than did the tumour cells in vitro, but both types of normal cells were similar to tumour cells in the hsp synthesized. Although the quantitative aspects of induction and repression of hsp may depend on cell type and microenvironment, the major tumour hsp being studied for function in vitro were qualitatively similar to those produced and labelled in vivo in response to a similar heat dose. Hsp were similar in both normal cells and tumour cells from the same host. These observations support the concept that hsp function in fundamental processes in the different microenvironmental and metabolic conditions found in vivo and in vitro. In addition, these observations suggest that prediction of tumour thermal response by measuring hsp levels may be influenced by host cell components. © 1987 Informa UK Ltd All rights reserved: reproduction in whole or part not permitted.