Target sites of adhesion-blocking univalent antibody fragments (Fab) were purified from membranes of Polysphondylium pallidum cells and their developmental regulation was studied. The major target site extractable with butanol-water proved to be a dimerizing glycoprotein. Monoclonal antibodies raised against this glycoprotein were used for antigen localization on the cell surface, and for affinity purification of the glycoprotein. Whereas the glycoprotein was present in both growth phase and aggregation-competent cells, a second target site of adhesion blocking Fab partially purified seemed to be a developmentally regulated protein. Quantitative data on Fab absorption by the two purified antigens were interpreted to indicate that more than one species of Fab must bind to the cell surface in order to substantially inhibit cell adhesion.