Glycophorin A and its isolated transmembrane region (GpATM) are each known to form sequence-specific dimers in SDS micelles. Whether this behavior accurately reflects behavior in red cell membranes or lipid bilayers, however, has remained unclear. Resonance energy transfer between labeled GpATM peptides has been used to observe dimerization of GpATM in bilayers. Separate populations of GpATM peptides were labeled with 2,6-dansyl chloride as the donor chromophore and dabsyl chloride as the acceptor. Quenching of the 2,6-dansyl chloride by the dabsyl group demonstrated an association of the labeled peptides. The quenching was not affected by increases in the amount of lipid present or by unlabeled heterologous peptides but was decreased by the addition of unlabeled GpATM. GpATM was determined to form dimers by fitting the observed energy transfer for a number of donor to acceptor ratios and fitting to the expected number of donor labeled peptides in an oligomer with an acceptor as a function of oligomer number. The finding that the GpATM peptide forms helical dimers in lipid bilayers supports the idea that GpA is a dimer in the erythrocyte membrane. The resonance energy transfer approach may extend to the study of other oligomeric complexes.