The low ionic strength extract of sheep erythrocyte ghosts has been fractionated by gel chromatography into: a complex of spectrin and actin, spectrin tetramer, spectrin dimer, actin, and hemoglobin. Spectrin heterodimer binds to F-actin but does not crosslink it. Spectrin tetramer can crosslink filaments of F-actin as shown by the large increase viscosity when the two proteins are mixed. The complex seems to induce the polymerization of G-actin to F-actin under condition where the actin would not have polymerized alone. Contrary to previous reports of others, the interactions of spectrin dimer and tetramer with actin are independent of the state of phosphorylation of the spectrin. Identical results were obtained with native spectrin (3-4 phosphates/dimer), spectrin to which 1 mole of phosphate was added by incubation with an erythrocyte protein kinase and ATP, and spectrin from which 85 percent of the phosphate groups were removed by incubation with E. coli alkaline phosphatase.