Fibroblast growth factors (FGFs) are related polypeptides with mitogenic activity on cells of mesodermal and neuroectodermal origin. The Fgf-3 gene shares high homology with FGF-2 and its protein product can substitute FGF-2 as a growth factor. Other observations, however, indicate that Fgf-3 has specialized functions. We have investigated the effect of the expression and secretion of Fgf-3 on the growth and transformation of the human breast epithelial cell line MCF-10A. Overexpression of Fgf-3 stimulates proliferation of these cells in serum-free medium and induces anchorage-independent colony formation in soft agar. In contrast, these effects were not observed with purified FGF-1 and FGF-2 on either the parental or the Fgf-3-MCF-10A cells. Thus, Fgf-3 is distinct from FGF-1 and FGF-2 for its ability to induce cell proliferation and transformation of MCF-10A cells. This difference could be due, at least in part, to the expression of a specific set of FGF receptors with higher affinity for FGF-3 than FGF-1 or FGF-2.