The present work was performed in order to clarify the effects of epidermal growth factor (EGF) and transforming growth factor-beta (TGF-beta) on the tumor growth of human breast cancer transplanted in nude mice. A local injection of 2 micrograms EGF significantly suppressed the tumor growth of human breast cancer MX-1 and primary breast tumor (UM-1) which was first isolated from a human female breast cancer patient in our laboratory. On the other hand, TGF-beta did not suppress the growth of those tumors. Our result indicates that human EGF may be useful as a therapeutic agent for some human breast cancers. Human EGF may lead to a new trend of therapy in the treatment of cancer.