Using the method of immunocytochemical staining in the reaction of indirect immunofluorescence we conducted a study of tumor cell of different etiologies on a limited number of patients using affinity purified antipeptide antibodies against monomeric (AT 19-36) forms of the protein B23 and antibodies against oligomeric forms (AT 283-294) that selectively stains isoform B23.1 compared to the commercial antibody against the protein B23 to determine their diagnostic and prognostic significance. As a result it was confirmed that the protein B23 known as an argyrophillic protein could also be used as a marker of malignancy of tumors when applying not only the immunohistochemical but also immunocytochemical method that is more accessible in the laboratory practice. It was established that not all molecule of the protein B23 is responsible for the informative value of the method but only the oligomeric form that marks the protein in the nucleoli of the tumor cells as multiple shining foci of high intensity as opposed to the monomeric form that is located in the cytoplasm and nucleoplasm with no luminescence present in the nucleoli. It was shown that the degree of expression of the isoform B23.1 that is revealed with the antipeptide antibodies AT 283-294 could serve as a new marker of the malignant tumors based on the quantity , form and intensity of staining of the nucleoli in the tumor cells.