The influence of membrane fluidity on hyperthermic cell killing has been investigated in ascites tumor cells. Membrane lipid composition of P388 ascites tumor cells were modified by feeding host animals with diets containing either unsaturated fatty acids (UFA) or saturated fatty acids (SFA). Both kinds of ascites were heat treated in vitro at 37, 42 or 43.5 degrees C for 30 or 60 min. The cell killing effect of hyperthermia was tested by transplantation of cells into recipient mice and survival examined. While at 37 and 42 degrees C for 1 h, there was no difference in cell killing of the two types of ascites, elevating the temperature to 43.5 degrees C the survival was significantly longer on transplantation of ascites of UFA diet. This effect was potentiated by membrane-fluidizing drugs. On the addition of 1 mM procaine during 1 h treatment at 43.5 degrees C, the ascites of SFA diet killed 80% of mice, while the ascites of UFA diet left all the mice alive at least for 3 months. Scanning electron-microscopic observations of the treated cells was performed in parallel and showed close correspondence with the results of survival studies. In conclusion, the increase of membrane fluidity by incorporating more of UFA or by the addition of membrane-fluidizing drugs or especially by the combination of both, the sensitivity of cells to heat enhanced. These experiments support the hypothesis that membranes are a target for hyperthermia.