One hundred and twenty-one rectal biopsies from 99 patients with carcinoma of the rectum or lower sigmoid colon were investigated using a high iron-diamine-Alcian blue technique for sulphated and non-sulphated acid mucins. It was found that in normal rectal mucosa sulphomucins are the main carbohydrate component of the goblet cell mucin. This normal ;mucous pattern' changes in the ;transitional' mucosa (histological normal mucosa adjacent to carcinoma) where there is an increase of non-sulphated acid mucins concomitantly with a decrease or absence of sulphated groups in 60 to 90% of the cases according to the type of tumour. The same type of changes in mucus were observed in the ;transitional' mucosa surrounding adenomatous polyps and papillary adenomas; they were not marked in areas around carcinoma in situ and not observed in the metaplastic polyps. These changes seem to be in direct relationship to the grade of differentiation and invasiveness of the tumour. The histochemical changes in the mucins seem to be in favour of a malignant potential in the so-called neoplastic polyps. The high iron diamine-Alcian blue, because of its ;specificity', consistent results, and easy technique is recommended for routine use together with haematoxylin and eosin staining in the diagnosis of premalignant changes.