Chemical modification of the chromatin DNA with alkylating derivatives of oligothymidylate (pT)16 and oligoadenylate (pA)16 bearing 4-(N-2-chloroethyl-N-methylamino)benzylphosphamide group at the 5'-phosphate has been investigated. It was found that the derivatives do react with DNA in chromatin. The reactions occur presumably at the complementary sequences of the DNA since the reaction of the oligothymidylate derivative is inhibited by oligonucleotide (pT)16 taken in excess and is not influenced by hexadecanucleotide of a random structure. Isolated DNA does not react with the oligothymidylate derivative. It is concluded that in chromatin, DNA is partially unwound or possesses some sites which can be opened easily in the presence of complementary oligonucleotides.