Isolated human erythrocyte spectrin is a dimer of two unique polypeptide chains. The dimer (alpha beta) undergoes reversible salt- and temperature-dependent association to form (alpha beta)2 tetramers. Spectrin also binds with high affinity to a protein receptor on the cytoplasmic surface of erythrocyte membrane vesicles. By cleavage of spectrin at its cysteine residues with 2-nitro-5-thiocyanobenzoic acid, a 50,000-dalton peptide fragment has been isolated which inhibits the binding of spectrin to erythrocyte membrane vesicles. This peptide arises from a terminal region of the beta chain. An 80,000-dalton peptide generated by restricted trypsin digestion binds preferentially to dimeric spectrin. This peptide arises from a terminal portion of the alpha chain. Multiple peptides involved in noncovalent associations between the chains have also been identified. These associations indicate that the two subunits of spectrin are aligned parallel to one another and that the tetramer formation site and the high-affinity membrane binding site are in close proximity to one another.