The submandibular gland is a rich source of epidermal growth factor (EGF) in mice. The concentration of EGF in the gland of virgin female mice of C3H/HeN strain increased as much as 9-fold from the age of 30 to 52 weeks. During this period, the incidence of mammary tumor in virgin females increased markedly to a maximal level of 62.5% (n = 48) at 52 weeks of age. Removal of the submandibular gland (sialoadenectomy) of virgin mice 14-22 weeks old reduced the tumor incidence to 12.8% (n = 39) at the age of 52 weeks and also increased the latency period of mammary tumor development as much as 14 weeks when compared to that of normal mice. Long-term treatment of sialoadenectomized virgin mice with EGF (5 micrograms per mouse every other day) increased the tumor incidence to 33.3%. Moreover, sialoadenectomy of mammary tumor-bearing animals caused a rapid and sustained cessation of tumor growth, but EGF administration (5 micrograms per mouse per day) quickly restored the rate of tumor growth to a normal level. These results indicate that submandibular gland EGF plays a crucial role in mouse mammary tumorigenesis.