Mammary tissue from five cows was cut into pieces approximately 3 mm2 x .2 mm and placed subcutaneously in athymic mice (10 to 12 pieces/mouse). After 30 d, all mice were injected for 10 d with 17 beta-estradiol (1 microgram), progesterone (1 mg), ovine prolactin (1 mg), and growth hormone (recombinant DNA-derived bovine) (1 mg). Four grafts were then removed from each mouse, mice were ovariectomized, and subsequently mice were injected daily for 7 d with hydrocortisone (.2 mg, all mice), growth hormone (0, .25, .5, or 1.0 mg), and 0 or .5 mg ovine prolactin (three mice/treatment per udder). alpha-Lactalbumin content of grafts was 36 +/- 9 micrograms/mg DNA after treatment with estradiol, progesterone, growth hormone, and prolactin but before hydrocortisone, prolactin, and growth hormone treatment. Hydrocortisone treatment increased alpha-lactalbumin to 147 micrograms/mg DNA. Growth hormone plus hydrocortisone treatment increased alpha-lactalbumin to 391, 451, and 480 micrograms/mg DNA for .25, .5, and 1.0 mg/d of growth hormone, respectively. Prolactin plus hydrocortisone treatment increased alpha-lactalbumin to 466 micrograms/mg DNA. With exogenous prolactin, growth hormone increased alpha-lactalbumin content of grafts to 581, 647, and 689 micrograms/mg DNA for .25, .5, and 1.0 mg/d of growth hormone, respectively. Histological examination of tissues indicated that the effect of growth hormone was not mediated through increased epithelial area. Data suggest that bovine mammary tissue is capable of organotypic differentiation in athymic mice.