8-(2-Anthroyl)-octanoic acid is a chromophore whose fluorescence properties are strongly dependent on the polarity of the surrounding medium. Synthesis and phase properties of anthroyiphosphatidylcholine molecules which were obtained by condensing this anthroyl fatty acid with egg lysophosphatidylcholine are described. In lipid bilayers, fluorescence quenching experiments performed with Cu 2÷ ions as quenchers show that the 2-anthroyi group is immersed in the depths of the membrane, at the expected position. In monolayers, anthroyl-PC exhibits a compression curve similar to that of anthryI-PC molecules also obtained by condensing 9-(2-anthryl)-nonanoic acid with egg lysophosphatidylcholine (J. de Bony and J.F. Tocanne (1983) Chem. Phys. Lipids 32, 105--121). These curves were similar to that recorded for dipalmitoylphosphatidylcholine, indicating that in monolayers, anthroyI-PC undergoes a liquid-expended to liquid-condensed phase transition. Molecular area of 0.44, 0.45 and 0.48 nm 2 were measured at ~r = 30 mN m "l for anthroyl-pc, DPPC and anthryl-pc, respectively. Fluorescence polarization and differential scanning calorimetry experiments show that in the form of liposomes or when fully hydrated, anthroyl-PC undergoes a gel to liquid phase transition at a temperature of 53°C (AT = 4 K). All these data indicate that in anthroyI-PC, the 2~anthroyl group is no more bulky than a normal acyl chain and that it does not affect the regular phospholipid molecular packing. Anthroyi-PC displays quite distinct fluorescence excitation and emission spectra when dispersed in water, alone, or in the form of a 0.5% (mol/mol) mixture with egg-PC, thus enabling the aggregated and diluted forms of the probe molecules to be identified. On these grounds, it is shown that anthroyl-pc exhibits good miscibility properties with lipids in the liquid phase (egg-PC, or dipalmitoylphosphatidylcholine (DPPC), distearoylphosphatidylcholine (DSPC) and dipalmitoylphosphatidylglycerol (DPPG) above T t) but not with lipids in the gel phase (DPPC, DSPC and DPPG, below Tt). Finally, fluorescence polarization experiments show that anthroyI-PC can be used for monitoring lipid phase transition. The potential use of this 2-anthroyl chromophore, when attached to a lipid molecule, for probing microenvironmental polarity and dynamics in membranes is discussed.