The combined effect of several sensitizers and light on H2O or D2O solutions of DNA-histone complexes, as well as the significance of singlet oxygen (1O2), in this photosensitizing reaction has been studied. On H2O solutions, the production of 1O2, as well as the formation of DNA-protein cross-links (DPCs), were found to be dependent on light dose for all the sensitizers. Mesotetra (4N-methylpyridyl) porphine (T4MPyP), methylene blue (MB), and toluidine blue (TB) were the best photosensitizers with regard to tryptophan photolysis, followed by hematoporphyrin (HP), thioflavine T (TT), and pyronin G (PG). The formation of DPCs showed high initial rates, reaching a plateau at doses over 90 J/cm2. Under these irradiation conditions, the percentage of DPCs induced by the sensitizers decreases in the order T4MPyP > MB > TB >> HP approximately TT >> PG (approximately 0). These DPCs were totally destroyed with proteinase K (15 micrograms/ml). The irradiation of the DNA-histone-sensitizer solutions in the presence of L-carnosine (5 x 10(-4) M) produced approximately a 50% of DPCs inhibition for T4MPyP, MB, and TB, and a total inhibition for HP, TT, and PG. The substitution of H2O by D2O as solvent significantly increased the photodegradation of tryptophan, as well as the photoinduction of DPCs by the sensitizers. The results obtained indicate that singlet oxygen is the main agent responsible in the DNA-protein cross-linking formation.