In this study we examined the expression of EGF receptor mRNA after EGF administration in hCG producing tumor (choriocarcinoma). We transplanted the tissue of choriocarcinoma into female nude mice and investigated the effects of EGF on the growth of tumors, the binding activity of EGF receptor and the expression of EGF receptor mRNA in the tumor tissues. Two doses of EGF 5.0μg, 50μg and phosphate buffered saline as a control were injected subcutaneously every day for four weeks. Removed tumors were used for immunocytochemical studies and EGF receptor mRNA investigations. HCG and EGF receptors were detected immunocytochemically in the tumor. The low dose EGF employed stimulated the tumor growth while the high dose EGF inhibited the tumor growth compared with that of the control group. The binding activity of EGF receptor and the expression of EGF receptor mRNA also changed in accordance with the stimulation or inhibition of tumor growth. Thegrowth of hCG producing tumor by EGF administration appeared to be dependent upon the binding activity of EGF receptor and the expression of EGF receptor mRNA.