Direct aggregation of synaptogenic cell adhesion molecules on the plasma membrane of dendrites and axons can induce the secondary clustering of pre- and postsynaptic proteins at sites of aggregation. For instance, direct clustering of neurexin-1 on the surface of axons by pre-clustered antibodies induces the recruitment of presynaptic vesicle markers including synapsin I. In this experiment, cell adhesion molecules are usually epitope-tagged in the extracellular region to provide a site for antibody recognition. Alternatively, antibodies raised against the extracellular region can be used for target protein clustering. This assay is similar to direct aggregation of cell adhesion molecules by antibody coated-beads. However, this method appears to give final immunofluorescence signals with lower background at least in our experimental conditions. We employed this assay to demonstrate that direct aggregation of NGL-3, a postsynaptic cell adhesion molecule, on dendrites induces the clustering of excitatory postsynaptic proteins including PSD-95 and NMDA receptors.