Fluorescence photobleaching recovery and immunofluorescence methods have been used to study the lateral mobility and topographical distribution of a major cell surface glycoprotein (CSP). Both endogenous CSP and fluorescent-labeled exogenous CSP bind to the cell surface in a fibrillar pattern and are immobile on the experimental time scale. Azide, vinblastine, and cytochalasin B do not alter the immobility and cell surface distribution of the CSP molecules. Therefore, oxidative phosphorylation and the cytoskeleton do not seem to be responsible for the properties of the bound glycoprotein. The presence of immobile CSP fibrils does not, however, impede the diffusion of a lipid probe, a ganglioside analogue, or various surface antigens. Therefore, the fibrils apparently do not form a "barrier" across the lipid phase of the plasma membrane. In contrast, concanavalin A binds to CSP and is largely immobile in regions rich in CSP. The presence of immobile concanavalin A receptors in areas or on cells lacking CSP indicates that other types of immobile concanavalin A receptors also exist.CSP does not bind to lipid bilayers composed of phosphatidylcholine or oxidized cholesterol. It does bind to dextran-coated bilayers as a diffuse distribution of mobile molecules that can patch after addition of antibodies to CSP. The latter result suggests that CSP molecules do not interact strongly with other CSP molecules under these conditions. Exogenous CSP binds to regions on the cell surface that already bear CSP. In view of the apparent weakness of CSP-CSP interactions on the lipid bilayer, it seems possible that the assembly of CSP fibrils is nucleated by cell surface components in addition to CSP.