It has been shown that phosphoprotein p53 may participate in the DNA-synthesis stimulation in the mixed culture of tumour cells and syngeneic embryonic mouse cells (10 days of pregnancy). This effect has been eliminated by the preliminary treatment of embryonic cells in vitro by the monoclonal antibodies to p53 (pAb421) in the model system. A conclusion is drawn that p53-epitopes of embryonic cell surface membrane take part in the formation of the determinant which is recognized by receptors of tumour cells. The stimulation of DNA-synthesis in tumour cells is resulted by this process.