In order to produce human growth hormone (hGH) in the milk of transgenic mice, two expression vectors for hGH differing in their 3' flanking sequences were constructed by placing the genomic sequences of hGH gene under the control of the rat beta-casein gene promotor. The 3' flanking sequences of the expression constructs were derived from either the hGH gene (pBCN1GH) or the rat beta-casein gene (pBCN2GH). Transgenic lines bearing pBCN1GH expressed hGH more efficiently than those bearing pBCN2GH in the milk (19-5500 micrograms/mL vs 0.7-2 micrograms/mL). In particular, one of the BCN1GH lines expressed hGH as much as 5500 +/- 620 micrograms/mL. Northern blot analysis showed that the transgene expression was specifically confined to the mammary gland and developmentally regulated like the endogenous mouse beta-casein gene in the mammary gland. However, a low level of nonmammary expression was also detected with more sensitive assay methods. In conclusion, the rat beta-casein/hGH fusion gene could direct an efficient production of hGH in a highly tissue-and stage-specific manner in the transgenic mice and the 3' flanking sequences of hGH gene had an important role for the efficient expression.