cDDP-resistant Ehrlich ascites tumour (EAT) cells (ER cells) were tested for cellular content of total glutathione, heat sensitivity, cDDP sensitivity and synergistic effects of a combined treatment of heat and chemotherapy. In comparison with the non-resistant EAT cells (EN) the ER cells had an elevated level of glutathione. Treatment with D,L-buthionine-(S,R)-sulphoximine (BSO), resulting in almost complete depletion of cellular glutathione, did not cause drug sensitization. The ER cells were somewhat less heat sensitive compared with the EN cells. Heat chemosensitization was observed for the EN cells as well as for the ER cells. At 43 degrees C (but not at 42 degrees C) the thermal enhancement ratio (TER) for cDDP toxicity was significantly higher in the ER cells. The total number of cells killed by the combined treatment was less in the ER cells than in the EN cells. After analysing existing literature, combined with the current results, it is concluded that although cDDP-resistant cells can often considerably be chemosensitized by hyperthermia, in most cases the difference in cDDP sensitivity cannot be overcome totally. In those situations where cDDP-resistant cells are more sensitive to heat and also show a high TER, especially at clinically relevant temperatures, hyperthermia as added modality is indicated for clinical treatment.