We attempted to develop a fluorescent probe superior to conventional ones (8-anilino-1-naphthalenesulfonate and auramine O) for use in the study of drug-binding sites on alpha 1-acid glycoprotein (AGP). It was found that quinaldine red (QR) strongly bound to AGP and had an enhanced fluorescence in the presence of AGP at a longer wavelength, although QR was rarely fluorescent in an aqueous or albumin solution. The binding parameters of QR to AGP were K: 1.3 x 10(6) M-1 and n: 0.9, using the fluorometric titration method. The fluorescence of QR in the AGP solution, however, was markedly quenched in the presence of basic drugs, indicating that these drugs competitively displaced QR from its binding site; the results were in good agreement with those in the literature. The good relationship between binding affinities and partition coefficients suggested that hydrophobic forces were involved in the binding of basic drugs to AGP. Moreover, the polarity of the binding site of AGP estimated from the relationship between the emission maximum of QR and Z values was 70, which corresponds to the same Z value of acetonitrile. These results distinguish QR from other conventional AGP probes as a better fluorescent probe by which to understand drug-AGP interaction and the characterization of binding sites on AGP in more detail.