The bovine prolactin vector was injected directly into the mammary glands of mice carrying the human transferrin transgene to investigate its effect on the production of human transferrin in milk. The mean levels of human transferrin in two experimental groups were increased by approx. 60% compared with the control group: 1143 +/- A 196 ng/ml (experimental group 1; two injections) and 1160 +/- A 189 ng/ml (experimental group 2; three injections) versus 714 +/- A 75 ng/ml (control group). These findings suggest the potential utility of the prolactin vector for efficient expression of valuable pharmaceutical proteins in transgenic animal mammary glands.