The thermotolerant kinetics of HEp-2 cells and its effect on cytotoxicity of Nitrocaphane (NC) were studied by colony formation assay. HEp-2 cells were exposed to two equal thermal doses (44 degrees C, 30 min) spaced at 0-24 hr intervals during which the cells were incubated at 37 degrees C. The maximum thermotolerance developed at the 8th hr and then gradually declined. Thermal dose survival curve even up to 2.5 hr of continuous exposure to 44 degrees C, no development of thermotolerance was demonstrated. If the cells were preheated at 44 degrees C for 30 min, and returned to 37 degrees C for 10 hr and then received the second heating at 44 degrees C for 3 hr, the slope of survival curve decreased (D0 = 2.26 hr). Thermotolerant ratio was 5.9. Thermosensitive HEp-2 cells were turned into thermotolerant cells by preheating at 44 degrees C, 30 min and returning to 37 degrees C for 10 hr. The thermotolerant cells showed resistance to drug or heat when they were exposed to NC or NC combined with heat.