The value of a number of in vitro systems which have been used in the investigation of carcinomas of colon and rectum is considered. Tissue culture cell ines which have been established from human and mouse rectal and colon tumours are described. Both mouse and human cells retain ultrastructural features of the tumours of origin; acinus formations, junctional complexes, microvilli with surface glycoprotein strands. Normal colo-rectal epithelium has not been established in cell culture but organ cultures of mouse colon, which retain normal ultrastructural features can be maintained for at least 28 days. The possible origin of non-epithelial cells which appear in cultures from normal colon is discussed. The areas in which the various in vitro systems may be of used are reviewed.