The LIM 1863 colon carcinoma cell line grows as structural organoids of goblet and columnar cells around a central lumen and provides a model for the development of stem cells in the normal colon. The organoid structure can be disrupted by removal of calcium from the medium, resulting in a suspension of single cells. Upon readdition of calcium, the cells reform the organoid structure over a period of 24 h, and ultrastructural examination of the reforming cells reveals that this involves a complex process that we have termed clutching. To determine the adhesion molecules involved in organoid formation we attempted to block this process by single cell suspensions of LIM 1863 reseeded in the presence of monoclonal antibodies. An anti-integrin antibody directed against a conformational epitope on the alpha v subunit totally inhibited organoid reformation. As a consequence of this inhibition of cell contact the colon carcinoma cells rapidly underwent apoptosis. Investigations of the apoptotic pathway involved suggested an induction mechanism since the onset of apoptosis in the contact-inhibited cells showed specific increased synthesis of 68- and 72-kD proteins. In addition, immunoblotting of cytosolic and nuclear extracts of the cells revealed the rapid translocation of the tumor suppressor gene product, p53 to the cell nucleus upon induction of apoptosis. These results suggest that cell-cell adhesion may be a vital regulator of colon development overcome in tumor cells by loss of adhesion molecules or of functional p53 protein.