DNA-protein cross-linkages were formed in isolated nuclei from immature and mature chicken erythrocytes by reaction with cis-diammine dichloroplatinum. On the basis of electrophoretic behaviour, the most abundant proteins involved in the cross-linking appeared to be present also in preparations of nuclear matrix. The maturation of the erythrocyte, which is accompanied by transcriptional inactivation, leads to a decrease in the amount of DNA-interacting proteins, to a loss of proteins capable of a specific recognition of DNA sequences and, unexpectedly, to the appearence of some new DNA-protein interactions. At least three cross-linked proteins were found predominantly or exclusively in nuclei of immature cells, and three others in those of mature ones. The three DNA-bound proteins, typical of mature erythrocytes, were not found among the components of a high-salt preparation of nuclear matrix. The results obtained suggest that, in addition to the well-known histone H5 and MENT protein, these newly identified DNA-bound proteins contribute to the formation of the condensed, inactive chromatin characteristic of mature erythrocyte.