To develop transgenic mice, a mouse metallothionein promoter–human growth hormone (Mt-hGH) gene fragment was injected into the pronucleus of C57BI × DBN/2J F2 one-cell embryos. Six founder animals grew larger (1.3-2.2 times) than littermate controls and had higher levels of hGH in plasma (4.6-279 mU/I). At 27-43 weeks of age, 3 of the 4 female transgenic founders developed malignant papillary adenocarcinomas of mammary origin. One of the male trantgenic founders was successfully mated and 2 of 3 female transgenic offspring developed mammary tumors. Forty-two female mice with the same genetic background as the transgenic animals and older than 43 weeks served as controls. No palpable tumor was found in this group. Five of the control animals were killed and examined histologically, revealing only normal mammary tissue. Earlier studies have shown that GH is important for growth and development of the mammary gland. Our study suggests that markedly elevated endogenous levels of GH cause mammary carcinoma in a specific strain of transgenic mice. The present animal model might be useful for studying molecular mechanisms that are involved in the development of hormonally induced mammary tumors.