The target site of adhesion-blocking Fab in aggregating Dictyostelium cells is a plasma membrane glycoprotein of approximate molecular weight 82,000 (refs 1--3), which is no longer synthesized after aggregation, and disappears from the plasma membrane. However, since cell adhesion remains important during later stages of differentiation, presumably important during later stages of differentiation, presumably another proteins(s) takes over its function. One candidate is a glycoprotein (molecular weight 95,000) whose synthesis commences at the tip stage and continues until the completion of development. Both proteins are strongly antigenic. When pseudoplasmodia ('slugs', a late developmental stage) are disaggregated and replated, the cells recapitulate the previous sequence of morphological changes much more quickly than before, although the glycoprotein of molecular weight 82,000 is not resynthesized during reaggregation. We report here that monovalent antibody (Fab) directed against slug plasma membranes inhibits reaggregation by binding to the glycoprotein of molecular weight 95,000 which thus seems to be involved in cell adhesion.