A part of the spectrin extracted from red cell membranes at low ionic strength occurs in the form of a high-molecular weight oligomeric complex with actin and proteins 4.1 and 4.9. When the extraction is performed at 35 degrees, the spectrin is present in this complex as the dimer, all higher forms being dissociated. We have been unable to establish any correlation between the fraction of the spectrin thus complexed and the metabolic state of the cell. At least a large part of the complex appears to be a defined monodisperse species, sedimenting at 31S. The actin is present as short protofilaments. The average number of spectrin molecules associated with each molecule of complex has been studied by cytochalasin binding and electron microscopy. The complexes present the appearance in the electron microscope of spiders, in which the legs are spectrin dimers, attached to a globular element, containing by inference, actin and proteins 4.1 and 4.9; they are active in nucleating the polymerization of G-actin. The complexes are extremely stable, being resistant to dissociation under the conditions of the deoxyribonuclease assay, even after treatment with trypsin to degrade the actin-associated proteins. It is suggested that the complexes represent intact junctions of the membrane cytoskeletal network. Relevant structural features of the network are revealed by electron microscopy. The results lead to inferences concerning the mechanism of dissociation of the network from the membrane.