An immunologically based method for the quantitative assay of molecules involved in cell adhesion is described. Three observations served as a basis for this assay: (a) cells obtained by trypsinization of retinal tissue aggregated rapidly, provided they had been allowed to recover in culture from the dissociation process; (b) treatment of chick retinal cells with Fab' fragments from rabbit antibodies against these cells prevented their aggregation; and (c) incubation of these antibody fragments with antigens released by retinal cells in culture neutralized their ability to inhibit aggregation. The amount of neutralizing antigen was determined by measuring the rates of cell aggregation in the presence and absence of antibody and antigen using a particle counter. Although adhesion was inhibited by anti-retinal cell antibodies, it was not affected by lectins or anti-carbohydrate antibodies that also were bound to the cell surface. Together, the results suggest that the inhibition involved blockade or inactivation of particular cell surface molecules and that the retinal cell antigens capable of neutralizing the antibodies represented these molecules or their fragments. In the accompanying paper, we describe the use of this assay for the purification from culture supernatants of a cell surface molecule involved in cell to cell adhesion.