The present study was aimed at investigating the spontaneous transfer of fluorescently labelled serine- and choline-glycerophospholipids from unilamellar vesicles to resting human blood platelets. The most effectively transferred phospholipids were pyrene-phosphatidylserine (PS) and the ether analogues of choline-glycerophospholipids, e.g., pyrene-alkylacyl- and pyrene-1'-alkenylacyl-glycerophosphocholines (plasmalogens). Transfer of pyrene-diacyl-glycerophosphocholine and pyrene-phosphatidic acid was almost not detectable under the same experimental conditions. The fast intermembrane PS-transfer could be explained by the very high degree of adsorption of PS donor vesicles to the platelet plasma membrane. The short halftime of transfer rate (12-14 min) and the high incorporation (1.08-2.16% of total platelet glycerophosphocholines) observed for ether choline-phospholipids in contrast to pyrene-PS (20 min, 0.8% of total platelet PS), could be interpreted in terms of their bulk membrane properties.