Modified phosphatidylcholine and sphingomyelin containing an anthryl end group attached to one of the fatty acyl chains were used as fluorescent probes in an investigation of the molecular organization of human high-density lipoproteins (HDL). Monolayer experiments and NMR measurements showed the anthryl-labeled lipids to mimic closely the corresponding host phospholipids, the fluorophores being located near to the terminal CH3 groups of the fatty acid residues. The above fluorescent phospholipid probes made it possible for the first time to study differentially the behaviour of phosphatidylcholine and sphingomyelin in HDL. The probes were shown to interact in a different way with the apoprotein tryptophans and to be non-randomly distributed at the surface of the globules. The probable sphingomyelin binding site of apolipoprotein A-I was defined. Evidence was obtained suggesting the existence in high-density lipoproteins of two slowly exchanging phospholipid pools: one strongly bound to apoproteins, and the other free or loosely bound. Fluorescence parameters characterizing the fluidity of HDL phospholipids and their interaction with the apoprotein tryptophans were found to correlate with the HDL cholesterol level. The possible significance of the obtained results for a better understanding of the relation of high-density lipoproteins to coronary heart diseases is discussed.