During in-vitro adhesion, spreading and proliferation of human endothelial cells (HEC) on tissue culture polystyrene (TCPS), cellular fibronectin is deposited onto the surface of TCPS in spite of the fact that relatively large amounts of proteins have been adsorbed from the serum-containing culture medium to this surface. Evidence is presented that serum proteins, adsorbed to the TCPS surface, are displaced by cellular fibronectin. In addition, the interaction of HEC with polyethylene, precoated with monoclonal antibodies directed against HEC membrane antigens and against extracellular matrix compounds, was studied. F(ab')2 fragments of two monoclonal antibodies were also included in this study. Preadsorption of these antibodies and F(ab')2 fragments resulted in cell adhesion and spreading as well as moderate cell proliferation (or no proliferation) for several days. A good cell proliferation of HEC was only observed on polyethylene precoated with fibronectin or an antibody directed against fibronectin. The results indicate that the direct or indirect deposition of fibronectin is a prerequisite for the proliferation of HEC. It is suggested that fibronectin, bound to a solid substrate, provides a biochemical signal necessary for the proliferation of HEC.