The effects of heat used either alone or in conjunction with 1-(4-amino-2-methyl-5-pyrimidinyl) methyl-3-(2-chloroethyl)-3-nitrosourea hydrochloride (ACNU) on cultured rat (C-6) and human (KY) glioma cell lines were quantitatively studied by colony formation assay. Flowcytometric analysis of the cell cycle progression of C-6 glioma cells after exposure to 43℃ was also carried out. The results obtained were as follows. 1) These cell lines had differences in heat sensitivity and in their response to the cytocidal effect of ACNU. 2) Synergistic effects of combined heat and ACNU were observed on both cell lines. 3) After exposure to 43℃ asynchronous C-6 cells accumulated initially in late S phase and later in the G2+M phase. These results suggest that the combined use of heat and ACNU is effective in the management of malignant tumors.