Assemblies are a specialization of intramembrane structure in astrocytes which is concentrated in astrocytic processes at the interface with vascular and cerebrospinal fluid compartments. When astrocytic processes are rapidly frozen and then freeze-fractured at very low stage temperature, the constituent particles of assemblies fracture in at least two planes in the lipid bilayer. The true outer surface of the astrocytic membrane can be exposed by etching rapidly frozen tissue and in such preparations the assemblies are seen to extend through the extracellular half of the membrane to be exposed on the surface to the extracellular fluid. The dimensions of the particles, their tendency to fracture at several levels and their exposure to the extracellular space all indicate that they are not composed solely of lipid. We conclude that assemblies represent a protein which partially or entirely bridges the membrane and which may serve a transport function.