The state of actin in the erythrocyte membrane cytoskeleton has been examined. The presence of ADP, rather than ATP, as the predominant nucleotide species reinforces the view that actin occurs in the polymerized form. Redeterminations of the amounts of the three cytoskeletal proteins, spectrin, actin and 4.1, present in the cell allow evaluation of some of the stoichiometric constraints on the construction of the cytoskeleton. The available evidence is compatible with a network consisting of spectrin tetramers as the structural members, attached at both ends to junctions consisting of 4.1 and short filaments (or 'protofilaments') of actin. Electron micrographs of isolated cytoskeletons support such a picture. The dynamic state of the actin has been studied, using the DNAase assay method. The results indicate that whereas native monomeric actin is present in the cell, only a small proportion of the subunits of the protofilament can enter into an equilibrium with this pool. A considerable proportion of the cytoskeletal actin is not liberated under conditions that dissociate F-actin and is evidently tightly associated with spectrin and 4.1. Attention is drawn to the possible consequences of ATP depletion on the state of the actin and thus of the cytoskeleton.