Mammary tissue from pigs on days 60, 80, 90, 100 and 100+ (days 106–111) of pregnancy has been cultured in vitro as explants. The total accumulation in tissue and culture medium of the whey proteins α-lactalbumin and β-lactoglobulin has been measured using specific radioimmunoassays. The control, uncultured tissue showed progressive morphological development from sparse, non-secretory epithelial tissue on day 60 to full lobulo-alveolar development with some accumulated secretion from day 100. In uncultured explants β-lactoglobulin could be detected consistently from day 90 (13 ± 12 ng/μg DNA, n = 4) and α-lactalbumin from day 100 (1·3 ± 0·5 ng/μg DNA, n = 11). At all stages of pregnancy, both whey proteins increased markedly during the period of culture (up to 7 d). Stimulation of α-lactalbumin appeared to be primarily under prolactin control. Prolactin increased α-lactalbumin accumulation to a similar extent alone, or in the presence of insulin and/or corticosterone. The response to prolactin was dose-dependent over the range 0·4–20 nM (10–500 ng/ml). Porcine prolactin was more potent than ovine prolactin. There was no effect of porcine growth hormone and no synergism detected between prolactin and tri-iodothyronine. By contrast, no specific hormonal requirements were established for accumulation of β-lactoglobulin, which appeared to increase in vitro if tissue remained viable in various combinations of insulin, corticosterone and prolactin. It was not stimulated by growth hormone. There was some indication of a prolactin-sensitive component in longer term cultures after day 4.