In order to study changes in complex carbohydrates of the cellular membrane and cytoplasm in neoplasia of large intestine, lectin-binding patterns were histochemically determined in 1,2-dimethylhydrazine (DMH)-induced rat neoplasia and human specimens using peroxidase-labelled lectins (PNA, WGA, UEA-1) with different sugar-binding specificities. The results obtained were as follows: 1) There was a difference in lectin-binding patterns among sites within the large intestine in rats. 2) There were differences in the lectin-binding patterns of PNA, WGA and UEA-1 between neoplastic and non-neoplastic mucosae in both rat and human distal large intestine. This result was considered to reflect the changes involved in the metabolism of complex carbohydrates accompanying malignant change. The cancerous cell surface showed a higher degree of binding than that of non-neoplastic mucosa, and irregular binding patterns of each lectin were characteristically observed in the cytoplasm. 3) UEA-1 showed the highest level of binding for human carcinoma, followed by adenoma and non-neoplastic mucosa, in that order, suggesting recognition of tumor-associated complex carbohydrates. 4) In the cytoplasm of mucosa adjacent to carcinoma, rat PNA and UEA-1 and human PNA showed high degrees of binding, probably expressing biochemical changes in cellular function that have not been observed in conventional morphological studies.