Transforming growth factor-alpha (TGF-alpha) stimulates while TGF-beta inhibits mammary epithelial cell growth, suggesting that when cells are treated concurrently with the growth factors their combined effects would result in no net growth. However, combined treatments stimulate proliferation and cellular transformation in several cell lines. The objective of this paper was to describe the effect of long-term (6 days) concurrent TGF-alpha and TGF-beta treatment on normal mammary epithelial cell growth pattern, morphology, and gene expression. Growth curve analysis showed that TGF-alpha enhanced while TGF-beta suppressed growth rate until Day 4, when cells entered lag phase. However, cells treated concurrently with both growth factors exhibited a dichotomous pattern of growth marked by growth and death phases (with no intermittent lag phase). These changes in growth patterns were due to a marked induction of cell death from Day 2 (16.5%) to Day 4 (89.5%), resulting in the transition from growth to death phases, even though the combined treated cultures had significantly more (P < 0.05) cells in S phase on Day 4. TGF-beta stimulated epithelial to mesenchyme transdifferentiation (EMT) in the presence of TGF-alpha, as characterized by increased expression of fibronectin and changes in TGF-beta receptor binding. Expression patterns of genes that regulate the cell cycle showed significant interaction between treatment and days, with TGF-beta overriding TGF-alpha-stimulated effects on gene expression. Overall, the combined treatments were marked by enhanced rates of cellular proliferation, death, and trans-differentiation, behaviors reminiscent of breast tumors, and thus this system may serve as a good model to study breast tumorigenesis.