To clarify the effect of L-histidinol on hyperthermic killing of cells, HeLa and mouse sarcoma-180 (S-180) cells were exposed to heat in vitro in the presence of L-histidinol and the clonogenic surviving fraction of the cells was examined. After pretreating the cells with L-histidinol for 4 h, exposure of the cells to the combination of heat at 43-degrees-C and L-histidinol for various times (30 min to 4 h) reduced the surviving fraction more prominently than heat alone. Optimal concentrations to confer effective enhancement of heat on HeLa and S-180 cells were 9 mM and 3 mM, respectively. Those concentrations showed little toxicity of L-histidinol alone. Enhancement of the effect of heat by L-histidinol was observed only at 43-degrees-C, but not at 41 and 42-degrees-C. Flow cytometric DNA analysis was used to examine the cell cycle transit effect of L-histidinol. L-histidinol alone arrested the HeLa cells in G1-phase. Heat treatment at 43-degrees-C led to an accumulation of the cells in G2/M-phase and a decrease in the G1-phase fraction. The effect of combined treatment with heat and L-histidinol was complementary, in which L-histidinol attenuated the accumulation of the cells in G2/M-phase and prevented a decrease in the G1-phase fraction. Thus, L-histidinol has the capacity to potentiate hyperthermic cell killing in vitro by a mechanism that may be related to the cell cycle transit effect.