Three fluorescent dyes (a merocyanin, a cyanin and a modified umbelliferon) have been incorporated into vesicles from three different lecithins. Their absorption spectrum, emission spectrum and fluorescence polarization in the lipid vesicles has been studied and compared with the spectral properties in homogeneous organic solutions. The reactivity of lipid bound dyes with reagents in the aqueous phase was investigated. The location use as indicators of phase transitions or membrane