HT-29 colon carcinoma cells form liver metastases upon intrasplenic injection, and adhesion to fibronectin under the liver microvascular liver endothelium is likely to be important for metastasis formation. We have therefore studied the integrins involved in fibronectin adhesion. This was not affected by blocking antibodies against the beta1, alpha3, and alpha5 integrin subunits, but it was blocked by an RGD-containing peptide, indicating involvement of RGD-dependent non-beta1 alphaV integrins. Both alphaVbeta5 and alphaVbeta6 were detected on HT-29 cells. Blocking mAb against alphaV, but not against alphaVbeta5, abolished adhesion. From a HT-29 cell lysate, only alphaVbeta6 bound to a fibronectin-Sepharose column. Thus, alphaVbeta6 is the main fibronectin receptor on HT-29 cells, despite the very low levels of alphaVbeta6 and the much higher levels of alphaVbeta5. The HT29 cells did not spread on fibronectin in the absence of serum, not even after a three- to fourfold increase in alphaVbeta6 levels, induced by interleukin 4. The cells did spread on vitronectin. Using immunofluorescence we observed that both on vitronectin and on fibronectin alphaVbeta5 was arranged in a striped pattern, aligned with actin fibers, and not in focal adhesions. On fibronectin, but not on vitronectin, alphaVbeta6 was concentrated in a punctate pattern at the periphery of cell islands.