Synaptic fractions of decreasing morphological complexity were prepared by phase partitioning of synaptic membranes in an aqueous two-phase polymer system containing increasing concentrations of the neutral detergent n-octylglucoside (OG). The morphology, distribution of concanavalin A binding sites (CABS) and protein and glycoprotein composition of the resultant fractions were examined. The lowest concentration of OG employed (0.5% w/w) gave fractions enriched in relatively intact junctions retaining both pre- and postsynaptic structures. Increasing the detergent concentration resulted in the stepwise solubilization of pre- and postsynaptic structures until purified postsynaptic densities (PSDs) were obtained with 1% (w/w) OG. CABS were generally distributed on all membrane structures present in the 0.5% OG fraction, were restricted to synaptic structures in the fraction obtained with 0.75% OG, and were localized to the convex (outer) surface of purified PSDs. Gel electrophoretic analysis showed that the restriction of CABS to the region of the synapse was associated with a marked increase in the concentration of glycoproteins with apparent molecular weights of 180,000 and 130,000. These glycoproteins were retained, and further concentrated in the purified PSD fraction.