Human lymphocytes pre-treated by heat (41℃ 1 h), low dose irradiation (50 mGy), or both low dose radiation and heat together, became less susceptible to chromatid damage induced by subsequent high dose of X-rays. Either heat or low dose X-rays could significantly reduced the yield of chromatid aberrations, but the magnitude of adaptive response was not enhanced when combined treatment with low dose radiation and heat were used. The results implied that the mechanism of adaptive response induced by low dose X-rays was as same as the mechanism of adaptive response induced by hyperthermia, which may be related with some protective proteins.