The patterns of mucosubstances secreted in the normal human colon and rectum, in benign tumours, and in and around malignant tumours of the colon and rectum have been studied. Very little neutral mucosubstance was seen in normal specimens; the secretion in these was made up of sulphated and nonsulphated acid mucosubstances, with the former predominating in the deeper half and the latter in the superficial half of the mucosa.Benign tumours contained moderate amounts of mucosubstance, predominantly acid nonsulphated mucosubstances. Differences in the pattern of secretion were observed between the adenomatous polyp on the one hand and the villous papilloma and the papillary parts of the papillary adenoma on the other. Carcinomas of colon and rectum contained very little secretion, with very scanty or no evidence of sulphation. The mucosa around carcinomas and in areas of non-polyploid hyperplasia in carcinoma-bearing colons showed two constant patterns, one consisting of very small amounts of secretion, with sulphated mucosubstance predominating, and the other showing normal or increased amounts of secretion almost entirely of the acid nonsulphated variety. The latter pattern extended for several centimetres from the tumour. The possibility that sulphated mucosubstance might have a role in controlling cell division and that a decrease in its secretion might initiate a sequence of changes leading to malignancy is discussed.