Synthesis of individual keratins (Nos. 8 and 19) was shown to continue in the simple epithelium of tumor cells by application of monoclonal antibodies in 20 cases of different patterns of human colonic malignancies. No correlation between the synthesis and degree of cataplasia of said cells was found. The increase in manifestations of cellular and structural anaplasia in tumor was matched by the enhanced intensity of cells' staining with antibodies. Application of said procedure in oncomorphological practice helps reliably evaluate the real extent of tumor spreading and detects invasion which other microscopic methods fail to do.