1. The catalyzed transfer of phosphatidylcholine from unilamellar liposomes to mitochondria by phospholipids exchange protein from beef heart or from beef liver is stimulated by the presence of up to 20 mol% acidic phospholipid (phosphatidylinositol or phosphatidic acid) in the liposome. Co-sedimentation of liposomes with mitochondria increases with increasing mol% acidic phospholipid. 2. The catalyzed transfer of phosphatidylcholine from unilamellar liposomes to multilamellar vesicles by beef heart or beef liver exchange proteins is also stimulated by the presence of acidic phospholipid. No co-sedimentation of negatively charged transfer of phosphatidylcholine from multilamellar vesicles to unilamellar liposomes by phospholipid exchange protein from beef heart or beef liver reaches a maximum at 7.5% phosphatidylinositol in the liposomes. Inhibition of phosphatidylcholine transfer was observed at levels of liposome phosphatiylinositol of greater than 15 mol% only in the presence of beef liver exchange protein. 4. Changes in the surface charge of liposomes by the addition of acidic phospholipid were verified by a novel application of polyvinylchloride block electrophoresis that allows the direct measurement of the relative electrophoretic mobility of sonicated vesicles.