The transfer of lipids between the endoplasmic reticulum and the Golgi apparatus was investigated in vitro using a cell-free system from leek seedlings. Lipids of the donor membranes (endoplasmic reticulum) were radiolabeled either by incubating leek seedlings with [1-14C]acetate or [3H]acetate. Acceptor membranes (Golgi apparatus) were unlabeled and immobilized on nitrocellulose strips. The assay measured the lipid transfer resulting from both an ATP-independent process and an ATP- and cytosol-dependent process. A significant ATP- and cytosol-dependent lipid transfer was observed only in the case of the endoplasmic reticulum as donor and the Golgi apparatus as acceptor. Lipids transferred in an ATP-dependent manner were chiefly phosphatidylcholine, phosphatidylethanolamine and phosphatidylserine. The stimulation of lipid transfer by ATP as compared to the ATP-independent process was +79% (PC), +123% (PS) and +69% (PE). On the other hand, PI was not transferred in an ATP-dependent manner (the stimulation by ATP was only 20%). This supports the theory that a sorting of phospholipids takes place in the donor membrane. Moreover, a formation of lysoPC was observed only in the presence of ATP (+330%). The ATP-dependent lipid transfer was inhibited by N-ethylmaleimide, indicating the involvement of cytosolic (but no phospholipid transfer proteins) or membrane proteins in the transfer process. The ATP-dependent transfer of lipids was also diminished at 12 degrees C showing the sensitivity to low temperatures of the transfer of lipids between the endoplasmic reticulum and the Golgi apparatus.