The physiological importance of EGF in the development of the mouse mammary gland during pregnancy and in spontaneous mammary tumorigenesis has been documented by a series of experimental results presented herein. In our study, we have taken a variety of experimental approaches including radioimmunoassay of EGF in the submandibular gland and plasma, mammary gland organ/cell culture, EGF receptor assay, sialoadenectomy and treatment with EGF and EGF antibodies to assess the role of EGF in the mammary gland. In particular, studies employing sialoadenectomy and EGF replacement have provided valuable information concerning the function of EGF in the body. These studies are possible in the mouse system because the submandibular gland serves as a major source of circulating EGF and also because purified mouse EGF is available commercially. Our work on the biological, endocrinological, and physiological aspects of EGF in normal and neoplastic growth of the mammary gland should be useful for the study of the regulation of mammary gland growth at the molecular level as well as for clinical investigations of mammary tumors. Finally, our findings of a mammary growth factor in embryonic mesenchymal cultures suggest the possible involvement of paracrine growth factor(s) in mammary cell growth. Further progress in this area is needed for better understanding of the complex process of mammary gland growth and development.