The effects of short chain fatty acids on a colon carcinoma cell line, LIM1215, have been studied. Of the four short chain fatty acids tested only butyrate at 1 mmol/l and 10 mmol/l and acetate at 10 mmol/l had significant effects on this cell line. The addition of butyrate to growth medium affected the growth rate and the production of alkaline phosphatase, dipeptidyl peptidase IV and carcinoembryonic antigen. Butyrate at a final concentration of 1 mmol/l increased the doubling time of the cells from 26 hours to 72 hours and decreased the cloning efficiency of the cells from 1.1% to 0.054%. Alkaline phosphatase concentrations increased rapidly in cells cultured in 1 mmol/l butyrate reaching peak levels after four days with alkaline phosphatase concentrations increasing more than six-fold. Levels of dipeptidyl peptidase IV and carcinoembryonic antigen were also increased after culture in butyrate containing medium. The number of alkaline phosphatase containing and dipeptidyl peptidase IV containing cells increased markedly in butyrate containing cultures. In contrast the number of mucus containing cells decreased in cultures grown in medium containing butyrate. This differentiating effect of butyrate on colon carcinoma cells may be relevant to the presence of butyrate in the colonic contents and the relationship between short chain fatty acids and fibre intake.