Activation of the receptor tyrosine kinase c-kit by the kit-ligand, also known as stem cell factor (SCF), is essential to melanocyte and germ cell development and during the early stages of hematopoiesis. Deregulated expression of c-kit has been reported in malignancies affecting these lineages, i.e., myeloid leukemias, melanomas, and germ cell tumors. In addition, c-kit and SCF are coexpressed in some breast and colorectal cancer (CRC) cells, raising the question of whether c-kit serves an autocrine role in normal or malignant epithelial tissues. In this study, we demonstrate that human colorectal carcinomas, but not normal colorectal mucosa cells, coexpress SCF and c-kit in situ. Expression of c-kit was also observed in mucosa adjacent to colorectal tumor tissue. Consistent with a growthregulatory role of SCF in CRC cells, exogenous SCF stimulated anchorage-dependent and anchorage-independent growth in four out of five CRC cell lines. Exogenous transforming growth factor (TGF)-b1 added at nanomolar concentrations to HT-29 CRC cells, which express the type I, II, and III TGF-b receptors, downregulated c-kit expression to background levels and inhibited c-kit-dependent proliferation. Similarly, TGF-b1 inhibited SCF-dependent proliferation of three firstpassage CRC cell lines. In summary, expression of the potential autocrine SCF/ c-kit axis is a tumor-associated phenomenon in colorectal cancer that can be suppressed by TGF-b1 in TGF-b-responsive CRC cells.