Although the colonic mucosa is one of the most rapidly proliferating epithelial tissues in the body, little is known about the factors that direct this proliferation. In this report we have studied the parameters of both a mitogenic and a clonogenic assay for detecting potential colonic growth factors (CGF). Using a colon carcinoma cell line (LIM 1215), which has retained a number of the properties of normal colonic mucosa, we have assayed a range of mitogenic factors for CGF activity. 3H‐thymidine incorporation by the LIM 1215 cell line was stimulated by low concentrations of epidermal growth factor and basic fibroblast growth factor and, to a lesser extent, by higher concentrations of interleukin‐I and insulin‐like growth factor I. The cells did not respond to a range of other mitogens and lymphokines. Optimal clonogenic response in a soft‐agar assay was obtained using a primary pituitary extract.