AbstractWide attention has been given to hyperthermia as a new measure for cancer treatment. Clinical trials of hyperthermia, as they possess antitumour activity on some occasions. When cells were incubated with oleic we examine if fatty acids exert a synergistic effect on Ehrlich ascites tumour cells when combined with hyperthermia, as they pssess antitumour activity on some occasions. When cells were incubated with oleic acid and linoleic acid (unsaturated fatty acids) at 37°C, the DNA synthesis of cells was significantly inhibited. Palmitic and stearic acids, which are saturated fatty acids, did not suppress DNA synthesis. Hyperthermic treatment without the presence of fatty acids at 42°C for 1 h decreased DNA synthesis to 62% of the control level of 37°C. The combination of an unsaturated fatty acid and hyperthermia synergistically suppressed DNA synthesis. When the cells were incubated in serum‐free medium containing 0.1 % albumin, unsaturated fatty acids were more effective in inhibiting DNA synthesis. However, saturated fatty acids had little or no effect on DNA synthesis in control or hyperthermia‐treated cells. These results indicate that unsaturated fatty acids are useful for enhancing the inhibitory effect of hyperthermia on DNA synthesis, which may increase thein vitroantitumour effects of hypert