An original physicochemical method is proposed for the evaluation of the photosensitizing activity of drugs in vitro. A Nuclear Magnetic Resonance (NMR) spectrum is recorded during light irradiation of drug solutions. The change in the intensity of the NMR lines under such conditions is termed the Photochemically Induced Dynamic Nuclear Polarization (Photo-CIDNP) effect. It is related to the formation of radical intermediates which may be involved in the in vivo photosensitization reactions (the so-called type-I photoreactions). Nine commercial quinolones were tested by this method: nalidixic, oxolinic, pipemidic and piromidic acids, rosoxacin, flumequine, enoxacin, pefloxacin and norfloxacin. Each quinolone was irradiated in alcoholic solutions in its UV absorption band (300-350 nm) in the absence or in the presence of a biological target chosen as a model: the amino-acid N-acetyltyrosine. The quinolones were classified in two groups in relation to the intensities of the observed CIDNP effects. Nalidixic and oxolinic acids, rosoxacin and flumequine are among the most potent photosensitizers.