Objective: The authors determined the molecular mechanisms for the failure of transforming growth factor beta (TGF-β) to inhibit the growth of SW1116 and SW48 colon cancer cell lines. Background: Transforming growth factor-β is a bifunctional regulator of cell growth that typically stimulates proliferation of mesenchymal cells, but inhibits proliferation of normal epithelial cells. In the colon, TGF-β appears to arrest proliferation of enterocytes as they leave the intestinal crypt and move to the villus tip. Transforming growth factor he actions are mediated by binding to heteromeric complexes of type I and type II TGF-β receptors. Loss of TGF-β responsiveness may contribute to uncontrolled cell growth and cancer. Methods: The effects of TGF-β1 on DNA synthesis were measured by incorporation of tritiated thymidine into DNA of cultures of moderately differentiated adenocarcinoma (SW48) and poorly differentiated adenocarcinoma (SW1116) colon cell lines and a mink lung epithelial cell line (CCL-64). The effects of TGF-β on the expression of c myc, TGF-α, and TGF-β in SW48 cells, SW1116 cells, and CCL-64 cells (c-myc only) were measured by Northern blot analysis. Expression of TGF-β receptors in the cell lines was measured using competitive binding assays, receptor affinity labelling techniques, and reverse transcriptase polymerase chain reaction. Results: Incubation with TGF-β1 (50 ng/mL) did not decrease serum-stimulated uptake of [3H]- thymidine into actively growing cultures of SW48 or SW1116 cells, but suppressed DNA synthesis of actively growing CCL-64 cells by 90%. Similarly, incubation with TGF-β1 (12 ng/mL) for 4 hours did not substantially alter the mRNA levels of c-myc, TGF-α, and TGF-β1 in either colon tumor cell line, although levels of c-myc mRNA in CCL-64 cells were reduced by TGF-β1 treatment. Competitive displacement of [125I] TGF-β1 binding detected high levels (16,500 TGF-β receptors per cell) of specific, high-affinity (200 pmol/L half-displacement) TGF-β receptors on CCL-64 cells. In marked contrast, very low levels of TGF-β1 binding to SW1116 cells (250 receptors per cell) and SW48 cells (260 receptors per cell) were detected. Autoradiograms of CCL-64 cells affinity labelled with [125I] TGF-β1 revealed the presence of type I, type II, and type III TGF-β preceptors. No TGF-β receptors were identified on SW1116 cells, and only very low levels of the nonsignaling type III TGF-β receptors were detected on SW48 cells. Reverse transcriptase polymerase chain reaction amplification detected mRNAs for type I, type II, and type III TGF-β receptors in CCL-64 cells, SW48 cells, and SW1116 cells. Conclusions: These results suggest that the lack of growth inhibition by TGF-β in SW48 and SW1116 colon cancer cells may be caused by a lack of expression of functional TGF-β receptors.