The ability of mammary tissue from prepubertal bulls and heifers of beef and dairy breeds to respond to hormonal stimuli through synthesis and secretion of milk proteins was studied. Experimental animals were six to eight month old Angus and Holstein cattle. All subjects were injected with estradiol and progesterone for seven days and slaughtered on day 15. Mammary tissue was explanted and cultured for 96 h in basal medium (B) which contained hormones necessary for maintenance, or stimulatory medium (P), further supplemented with prolactin. Selected cultures were incubated for 24 h in B or P medium containing 3H-amino acids. Concentrations of non-labeled alpha-lactalbumin (Alac), 3H-Alac, and 3H-total protein (TP) were determined in media and in explant homogenates. Among cultures of bull mammary tissue, Angus explants secreted greater overall quantities of 3 H-TP and 3H-Alac than Holstein explants (p<.05). Secretion of Alac was also greater in Angus cultures at two of eight treatment periods (p<.01). Concentrations of all three protein fractions were likewise enhanced in homogenates of Angus explants for at least three of four treatment periods (p≤.05). Presence of prolactin in medium stimulated secretion of Alac (p<.005), and accumulation of all three fractions in explants (p<.10). Holstein heifer explants secreted more Alac at three of eight treatment periods than Angus explants (p<.0005). Overall secretion of ³H-TP and ³H-Alac also was elevated in Holstein over Angus females (p<.10), as were concentrations of all three fractions in homogenates (p≤.01). Presence of prolactin had no direct effect on any protein parameters in female tissue. I conclude that mammary tissue of immature bulls and heifers can be hormonally induced to express it's genetic merit for milk production (based on breed differences), through synthesis and secretion of milk proteins. Prolactin stimulated protein production in bulls but not in heifers. These findings indicate that similar methods of stimulating mammary tissue to produce milk proteins may be adaptable for commercial evaluation of genetic potential for milk production, especially in young bulls. / M.S.