The effects of the photodynamic action of methylene blue (MB-PDA) on strains of Escherichia coli were investigated to determine whether the dye could be used in photodynamic therapy (PDT). Using the method of alkaline sucrose gradient sedimentation, it was shown that in darkness MB induces a type of prelesion in DNA that transforms into single-strand breaks in alkaline conditions, provided that the dye is present during the processing of the gradient. This prelesion is completely reversible if the cells are washed immediately to remove the dye. However, after illumination with white light, the prelesions become "fixed" stable lesions, irreversible even after successive washings. The lethal damage induced by MB-PDA in E. coli can be repaired by the excision-repair system (about 30%) and by the recA-dependent repair system (about 70%). Polymerase I enzyme participates actively in the repair of the damage. MB-PDA is a weak mutagen and the induction of mutations by this treatment is restricted to high survival rates. Moreover, MB-PDA does not induce the SOS system (an inducible repair system dependent of the recA and loxA genes products), as measured by Weigle reactivation. However, it seems that this treatment can impair the repair systems in E. coli.