In colitis, colonic epithelial cells have a shortened life span but show normal or increased expression of phenotypic markers of differentiation. This study examined the effect of differing culture conditions on the expression of such markers in colonic crypt cells. Crypt cells were enzymatically isolated from macroscopically normal large bowel mucosa resected because of neoplasia, inflammatory bowel disease or non-neoplastic non-inflammatory conditions. Cells cultured in the presence of serum exhibited a doubling of the rate of protein synthesis (measured by 14C-leucine uptake; p<0.001) compared with autologous cells cultured in the absence of serum without evidence of loss of cell viability (assessed by 51Cr release from prelabelled cells) or of change in the rate of cell proliferation (assessed by total DNA content and 3H-thymidine uptake). Irrespective of the underlying colonic disease, crypt cells cultured in the absence of serum exhibited increased expression of phenotypic markers of differentiation compared with those cultured with serum: th rate of glycoprotein synthesis relativ to that of protein synthesis increased by a mean of 59% and the cellular expression of brush border glycoproteins, alkaline phosphatase, and carcinoembryonic antigen significantly increased. The effects seen could not be mimicked by addition of dexamethasone or insulin to serum free medium. Thus, under less optimal (serum free) culture conditions, colonic crypt cells express phenotypic markers of differentiation at an accelerated rate suggesting that unfavourable microenvironmental conditions themselves are probably in part responsible for the normal or increased expression of such markers in colitis.