Two proteins, one in a highly purified form, have been isolated from the soluble fraction of rat liver homogenate. These proteins accelerate the transfer of labeled phosphatidylethanolamine, phosphatidylcholine, phosphatidylinositol, sphingomyelin, and cholesterol from liposomes to mitochondria or erythrocyte ghosts. The fraction obtained after ammonium sulfate precipitation, gel filtration on Sephadex G-75, ion-exchange chromatography on CM-cellulose, ampholyte displacement chromatography, and heat treatment exhibited an 876-fold increase in its phosphatidylethanolamine transfer activity as compared with the postmitochondrial supernatant adjusted to pH 5.1. Isoelectric focusing on polyacrylamide gels shows a single band between pH 8.6 and 9.0. The transfer activity is abolished by trypsin, but withstands 5-min heating at 90 degrees. After heat treatment, a single major band is seen on polyacrylamide gel electrophoresis followed by two minor ones. The molecular weight of the major band is 12,500, as determined by electrophoresis on 15% polyacrylamide gels in the presence of sodium dodecyl sulfate. A molecular weight of 13,500 was calculated from molecular filtration through Sephadex G-50. The relative transfer activities toward the different phospholipids remain constant throughout the last three steps of the purification procedure in spite of the extensive change in the electrophoretic profile of the protein mixture. The cholesterol transfer activity remains unchanged after the final heat treatment as well. This indicates that all of the transfer activities are present in a single protein.