A new method for covalent binding of histones to partially apurinized DNA was developed. Partial apurinization of DNA methylated within the composition of chromatin results in a formation of aldehyde groups interacting with the epsilon-amino groups of chromatin proteins lysine residues. The resulting Schiff's bases covalently and reversibly bind the protein molecules to DNA. This covalent binding is accompanied by a specific one-chain cleavage of DNA at the cross-linkage point in such a way that only the newly formed 5'-terminal fragment of DNA in bound to the protein. These cross-links can be stabilized via reduction of Schiff's bases by sodium borohydrate. Determination of the size of the bound DNA fragment allows to establish the localization of the cross-linkage point and the position of the protein molecule on DNA. The method of cross-linkage with a "zero length" allows to fix the immediate DNA--protein interactions and can be extensively used to study the protein--DNA interactions in cases when the epsilon-amino groups of protein lysine residues interact with DNA.