The flexibility of the human erythrocyte membrane is mediated by an underlying network of skeletal proteins which interact with the membrane through ankyrin and protein 4.1. The nature of the membrane attachment site(s) for protein 4.1 has yet to be fully elucidated. In this paper we show that purified protein 4.1 binds much less strongly to alkali-stripped membranes from erythrocytes of individuals with total glycophorin C and D deficiency (Leach phenotype) than to alkali-stripped normal membranes. We further show that a synthetic peptide corresponding to amino acid residues 82-98 of the cytoplasmic domain of glycophorin C specifically binds to purified protein 4.1 and inhibits protein 4.1 binding to alkali-stripped normal membranes. The same synthetic peptide binds directly to membranes from individuals with glycophorin C and D deficiency but not to normal membranes. These results indicate that glycophorins C and D provide major membrane attachment sites for protein 4.1 in normal erythrocytes and that this interaction is mediated by protein 4.1 binding to amino acid residues 82-98 of glycophorin C and 61-77 of glycophorin D.