A head-group modified, photoreactive analog of phosphatidylethanolamine, N-([125I]iodo-4-azidosalicy- lamidyl)-1,2-dilauryl-sn-glycero-3-phosphatidylethanolamine ([125I]-N-ASA-DLPE), has been used in photoaffinity labeling studies of proteins of the human erythrocyte membrane. [125I]-N-ASA-DLPE was shown to be preferentially incorporated into a protein with an apparent molecular weight of 31 kDa. Protein sequencing and immunoprecipitation were used to identify this protein as the erythrocyte membrane protein, band 7.2b or stomatin. A sulphydryl-reactive ligand, 4-hydroxy-3-(iodo-[125I])-N-[2-(2-pyridinyldithio)ethyl]- benzenepropanamide ([125I]-PDA), was also shown to preferentially label band 7.2b. We propose that band 7.2b may act as a site of transbilayer reorientation of membrane phospholipids.