The formation of DNA-protein cross-links (DPCs), induced by irradiation with visible light, was studied in methylene blue-treated (MB-treated) chromatin and H1-depleted chromatin. The effects of the MB concentration and radiation dose were studied using sodium dodecylsulphate-chloroform-isoamyl alcohol assay and sodium dodecylsulphate-polyacrylamide gel electrophoresis. Under identical experimental conditions, DPC formation was less in H1-depleted chromatin (70%) than in chromatin (92%). The non-histone proteins and core proteins of chromatin contributed towards DPC formation. Of the core proteins, H2A was more cross-linked than H4, whereas the bands for H2B and H3 melted into one in chromatin and H1-depleted chromatin. In both cases, the gel pattern showed the appearance of two new protein bands with approximate molecular weights of 27 kDa and 29 kDa as a result of histone-histone cross-linking. Viscometric studies showed that the dissociation of the compact structure of chromatin in 2 M NaCl was more extensive in irradiated, MB-treated, H1-depleted chromatin than in irradiated, MB-treated chromatin, indicating a reduction in the amount of DPC formation in H1-depleted chromatin.