To evaluate the bioactivity of bovine milk from different stages of lactation on human intestinal tissue, a human fetal small intestinal cell line was used as a model system. Milk samples representing six stages of lactation: days 1, 2–3, 6–7 and weeks 12 and 24 after parturition, 1 week before drying off, and milk-like secretion from two stages of the dry period: 7 weeks and 3–4 weeks before expected calving, were collected from 64 Holstein Friesian cows. The whey fraction of the milk or milk-like secretion was added to the culture medium in concentrations ranging from 0·078% to 10%. The growth-promoting activity of whey was measured by determining the incorporation of [3H]thymidine into DNA for the last 24 h of the culture period. Whey fractions from all six stages of lactation stimulated growth of intestinal cells. The growth-promoting activity of colostrum or milk significantly decreased within the first week after calving. The growth-promoting activity in mature milk increased gradually during lactation to reach a level significantly higher than that obtained with colostrum. The growth-promoting activity of whey from milk-like secretion collected after drying off was lower than that of colostrum. Whey from different stages of lactation contained significantly different concentrations of TGF-β1 (0·5–27 ng/ml) and TGF-β2 (12–1219 ng/ml). However, neither the differences in TGF-β1 and TGF-β2, nor the differences in IGF-I and IGF-binding proteins could fully explain the differences in growth-promoting activity of colostrums or milk from different stages of lactation, suggesting that other factors were also involved. The present study showed that bovine milk contained a number of biologically active components that affected growth and development of human intestinal tissue. The results showed that the growth-promoting activity of colostrum and milk was dependent on the stage of lactation in accordance with previous results obtained with mammary epithelial cells. The changes in growth-promoting activity with stage of lactation were probably related to changes in concentrations of several growth factors.