Production of milk proteins in response to hormonal stimuli was studied in organ culture of mammary tissue from prepubertal Angus or Holstein bulls. Bulls were injected with estradiol and progesterone for 7 d and slaughtered on d 15. Mammary tissue explants were cultured for 96 h in basal medium containing insulin, hydrocortisone, and triiodothyronine or in stimulatory medium, further supplemented with prolactin. Selected cultures were incubated in the presence of [3H]-amino acids. Content of alpha S1-casein in cultures from Angus bulls was increased 3-fold in medium, and 6-fold in explant homogenates, relative to cultures from Holstein bulls. Concentration of 3H-labeled protein was greater in medium (2.3-fold) of Angus versus Holstein cultures. Overall, alpha-lactalbumin content in medium and homogenates tended to be higher in Angus cultures. Prolactin increased alpha-lactalbumin and casein in medium, and alpha-lactalbumin and [3H]-protein in homogenates. We conclude that mammary tissue of immature bulls can be induced to produce milk proteins and that prolactin enhances production. Genetic (breed) differences in responses were observed, although the greater productivity of explants from Angus compared with explants from Holstein bulls was unexpected. Nevertheless, use of measurements such as these may provide data for sire selection.