AbstractPhysical studies and conformational analysis of human glycophorin A suggest a revised model for its molecular organization, self‐association, and interactions with the erythrocyte membrane. Intrinsic viscosity has been used to study, under more physiological conditions, the monomer–dimer equilibrium demonstrated previously by polyacrylamide–SDS gel electrophoresis. The results show that the equilibrium persists in the absence of detergent and support earlier indications that the dimer is probably the physiologically relevant form and that it is promoted by salt, inhibited by conventional denaturants, and abolished by carboxymethylation.Combined application of CD, fitted to the poly‐(L‐lysine) model spectra of Greenfield and Fasman, and conformational prediction, by the statistical method of Chou and Fasman and the stereochemical approach of Lim, suggests five helical sequences in glycophorin A: Arg‐39 to Tyr‐52 (A); Gln‐63 to Glu‐70 (B); Glu‐72 to Leu‐89 (C); Ile‐95 to Lys‐101 (D); and Leu‐118 to Asn‐125 (E). Sequence A occurs only at low pH and may be stabilized by favorable noncovalent interactions of O‐linked tetrasaccharide side chains. The other four helices all occur in the dimeric form of glycophorin A at physiological pH and ionic strength. Sequence D is destroyed by trypsin, and is also lost on conversion to the monomeric form of the glycoprotein at low ionic strength. Sequence E is denatured by 6M guanidine hydrochloride/4M urea. Sequences B and C, which are separated by a single proline residue, are stable under all these conditions.Dimerization of the major, hydrophobic helical sequence, (C) may be promoted and directed by an adjacent short sequence of intermolecular parallel β‐sheet (Leu‐90 to Tyr‐93). It is proposed that these two structures span the lipid bilayer in vivo, and that helices B and D lie, respectively, along the outer and inner surfaces of the membrane. Molecular organization in the N‐ and C‐terminal regions of the molecule is discussed in terms of evidence from the present work and from other recent investigations.