Loss of O-acetyl substituents from sialic acid expressed in mucin secreted by hyperplastic polyps (21), adenomas (9), a mixed polyp (1) and adenocarcinomas (41) of the colorectum was investigated by mucin histochemistry (diastase PAS and mild PAS) and by lectin histochemistry (Arachis hypogaea or peanut agglutinin) with (nPNA) and without (PNA) prior neuraminidase digestion. Mild PAS and nPNA reactivity were closely correlated, indicating that loss of O-acetyl substituents at C7, C8 and C9 (hence mild PAS positive) and at C4 (hence neuraminidase labile) occur pari passu. These sialic acid alterations were characteristic of mucin secreted by both adenocarcinoma and hyperplastic polyp. The same changes occurred patchily or focally in adenoma. Five "serrated" adenocarcinomas resembled the hyperplastic polyp both morphologically and histochemically. Luminal secretions within cancers were classified as mucin-like (type I) and non-mucin-like (type II). Mild PAS was the most specific technique for mucin-like intraluminal material. However, accumulated luminal secretions (type I or II) and intracytoplasmic lumina were quite specific features of colorectal cancer and could be effectively highlighted by means of dPAS. PNA reactivity without prior neuraminidase digestion showed a distribution unlike nPNA. Whilst PNA expression was more cancer specific than either mPAS or nPNA, it was observed mainly in cancers secreting little or no mucus, thus limiting its value as a tumor marker.