The secretion of transforming growth factor alpha (TGF alpha) and the expression of cell-surface receptors for epidermal growth factor (EGF) were measured in a series of human mammary cell lines. The amount of TGF alpha secreted by the cells did not correlate with the phenotype of the cells (epithelial or myoepithelial), the mechanism of immortalization of the cells (SV40 or spontaneous) or the source of the cells (normal mammary gland, benign hyperplastic lesion, malignant tumour). The level of expression of cell-surface receptors for EGF was markedly increased as a consequence of SV40-immortalization of mammary cells, but otherwise did not correlate with the phenotype of the cells or the source of the cells. Much of the increase was accounted for by the appearance of a large number of low-affinity receptors for EGF in the SV40-immortalized cells. It is suggested that one of the mechanisms whereby SV40-immortalization suppresses the senescence of primary cultures of human mammary epithelial cells involves increasing the level of expression of receptors for EGF. In contrast the level of secretion of TGF alpha by cells in culture is probably a consequence of the mechanisms of adaptation of each cell line to culture conditions, and does not reflect the level of secretion of TGF alpha by cells in vivo.