Interaction of bilirubin with phospholipid liposomes was studied at varying pH. Liposomes were prepared from egg-phosphatidylcholine, dipalmitoyl-phosphatidylcholine, phosphatidylethanolamine, egg-phosphatidylcholine, and phosphatidylethanolamine with 5% (w/w) cholesterol, phosphatidylserine, sphingomyelin, and a lipid preparation from cat brain. Experiments were also conducted with erythrocyte ghosts. Interaction with bilirubin was studied by observing quenching of fluorescence from 1,6-diphenyl-1,3,5-hexatriene, incorporated in the lipid bilayers, by gradient centrifugation of the product, by measuring light scattering during the process, and by studying the solubility and infrared spectra of the final aggregates. At pH values above 8.2, the findings are consistent with formation of a bilirubin dianion-phospholipid complex. At pH 6-8, protonization of bilirubin in the complex is indicated and is followed by self-aggregation of bilirubin acid with formation of large bilirubin particles containing a small amount of phospholipid. Results were qualitatively similar with all lipids studied; marked quantitative differences in degree of fluorescence quenching and aggregation rates were observed with liposomes from different phospholipids. Reflections on affinities and available concentrations indicate that this train of processes may be part of the toxic mechanism of bilirubin.