The solubility of the dye-detergent complex, Coomassie brilliant blue-cetyltrimethylammonium bromide (CBB-CTAB), in various phospholipids vesicles and in amniotic fluid phospholipid was determined as well as the aqueous/organic distribution ratio of the solubilized products in an aqueous-organic two-phase system. The solubility of the complex depended on the phospholipid head group and decreased in the order: ((\mathrm{PG}) \approx \mathrm{PI} \approx) (\mathrm{PS}>\mathrm{PC} \approx \mathrm{DPPC}>\mathrm{PE}>\mathrm{SM}) (PG, phosphatidylglycerol; PI, phosphatidylinositol; PS, phosphatidylserine; PC, phosphatidylcholine; DPPC, dipalmitoylphosphatidylcholine; PE, phosphatidylethanolamine; SM, sphingomyelin). Dye solubilized in anionic phospholipid vesicles distributed to the aqueous phase, while that solubilized in zwitterionic phospholipid vesicles distributed to the organic phase. CBB solubilized in a binary mixture of lecithin and anionic phospholipid ( (\mathrm{PG}, \mathrm{Pl}) ) vesicles containing more than (30 \mathrm{~mol} %) of the anionic phospholipid distributed preferentially to the aqueous phase. On the other hand, dye solubilized in (\mathrm{PC} / \mathrm{Pl}) (more than (30 \mathrm{~mol} % \mathrm{PI}) ) vesicles containing SM and in :mnioric fluid phospholipid distributed proportionally (o) the agucous and organic phases. In both lipid mixtures, the dye products partitioning into the aqucous phase formed before those partitioning into the organic phase. The data indicated that the association between the dye complex and phospholipid is sensitive to the phospholipid composition of the vesicles. (1) 1993 Academic Press. Inc.