Ligand-binding studies were performed with a basic 23 kDa protein purified from bovine brain cytosol. By equilibrium dialysis experiments bromosulfophthalein, dehydroepiandrosterone sulfate and oestradiol-17 beta were demonstrated to bind to the protein with association constants of 1 X 10(6), 1 X 10(4) and 1 X 10(3) l/mol, respectively. Indocyanine green, Evans blue and Rose Bengal were not bound. The protein was further characterized as a phosphatidylethanolamine-binding protein, while phospholipid transfer assays proved negative. The so far investigated binding characteristics of the 23 kDa cytosolic protein, together with previously demonstrated sequence homologies with other known cytosolic proteins, suggest its involvement in lipid metabolism.