The aggregation of cells from dissociated neural retinas of chick embryos can be inhibited by antibodies prepared against whole retinal cells. In order to identify the antigens involved, substances released by retinal tissues in culture were tested for their ability to neutralize specifically the inhibition by antibody of cell adhesion. Using this assay, three active polypeptides from the culture supernatant were purified 500-fold by gel filtration and polyacrylamide gel electrophoresis. Rabbit antibodies prepared against these purified supernatant activities inhibited cell adhesion and reacted only with the three polypeptides. Immunoprecipitation by the specific antibodies of 3H-labeled proteins from a detergent extract of embryonic retinal cell membranes yielded a polypeptide having a Mr of 140,000 in sodium dodecyl sulfate. This precipitation was inhibited in the presence of the three culture supernatant polypeptides that had activity, suggesting that they contained antigenic determinants in common with the 140,000 Mr surface component. They therefore represent all or parts of this cell surface molecule that were released into solution during tissue culture. The data are consistent with the hypothesis that the 140,000 Mr polypeptide is intimately involved in initial adhesion among neural cells.