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Summary form only given. It is well known that 3-hydroxyflavone exhibits the solvent-controlled excited-state intramolecular proton transfer (ESIPT) reaction and therefore its derivatives can serve as sensitive fluorescent probes for investigating microparameters of biological membranes and macromolecules. We present results of steady-state and time-resolved fluorescence spectroscopy investigations of 4'-diethylamino (FET) and 4'-(15-azacrown-5) (FCR) derivatives of 3-hydroxyflavone in simple and binary aprotic solvents, and human erythrocyte membranes. The presence of the 4'-amino substituent in the 3-hydroxyflavone molecule leads to an increase in its dipole moment, which results in more pronounced solute-solvent effects. Due to the different sizes of substituents in the FET and FCR molecules it is expected that these compounds can be used as fluorescent probes sensing different regions of a biological membrane.