The photodynamic action of bilirubin on isolated human erythrocyte membranes (ghosts) has been studied. When incorporated into ghosts (pH 8.0,10 degrees) the bile pigment photosensitizes in blue light the peroxidation of unsaturated lipids, as evidenced by a positive color reaction with 2-thiobarbituric acid. Accompanying lipid peroxidation was the disappearance of most of the major membrane proteins (Coomassie Blue staining in sodium dodecyl sulfate-polyacrylamide gel electrophoresis) and appearance of polypeptide photoproducts of greater size (mol wt greater than 250,000). The association of membrane proteins (presumably by cross-linking) was insignificant when bilirubin-ghost suspensions were kept in the dark, or when ghosts were irradiated in the absence of bilirubin. Electrophoretic bands 1 and 2 (Fairbanks, G., Steck, T.L., and Wallach, D. F.H (1971), Biochemistry 10, 2606) diminished rapidly during the photoreaction, whereas band 3 and the three sialoglycoproteins disappeared at a much slower rate. Dispersal of membrane consituents by treatment with sodium dodecyl sulfate prior to irradiation resulted in relatively little peroxidation and no noticeable formation of high molecular weight polypeptide complexes. The possibility that malonaldehyde, a product of lipid peroxidation, is involved in cross-linking during irradiation was studied by incubating ghosts with exogenous malonaldehyde. Although the reagent did cross-link membrane proteins (electrophoretic bands 1, 2, 2.1 2.2, and 4.1 diminished most rapidly and high molecular weight bands appeared), the reaction could only be demonstrated with malonaldehyde concentrations several orders of magnitude greater than those detected in irradiation experiments. If malonaldehyde cross-linking occurs, it does not appeare to be the predominant mechanism of polypeptide association during irradiation of bilirubin-containing ghosts.