We examined 35 cases of stomach carcinoma and 40 cases of colonic carcinoma with PNA associated with peroxidase (peanut agglutinin, lectin which binds to the terminal disaccharide galactose beta (1,3)-N-acetil-galacto-samine). In this way evaluation of the functional aspects of the normal-neoplastic sequence was undertaken. This method was carried out for histological and ultrastructural investigations. The results obtained in both cases showed a different reactivity in the evolution of neoplastic disease: in fact, positivity in dysplasia is finely granular intracytoplasmic, whereas in well-differentiated neoplastic transformation such a reactivity is preferentially localized along the cellular membranes, with restoration of gross positivity in the cytoplasm for the poorly-differentiated neoplasm. We therefore believe PNA to be a marker not only of neoplastic progression but of differentiation as well: we also hypothesize it to reveal glycoprotein groups with possible antigenic power, involved in immunologic interactions between tumor and host.