1. This study investigates the acetylation of 5-aminosalicylic acid by isolated human colonic epithelial cells. 2. After incubation of intact cells with 0.1 mmol/l 5-aminosalicylic acid, N-acetyl-5-aminosalicylic acid was detected in a concentration of 141 (+/- 23.8 SEM) nmol/g dry weight in the incubation medium, and 34.8 (+/- 5.5 SEM) nmol/g dry weight intracellularly. No unchanged 5-aminosalicylic acid was detected inside the cell. 3. Acetylation of 5-aminosalicylic acid by a cell homogenate was very poor, but the addition of 1 mmol/l acetyl-CoA resulted in complete conversion of 0.1 mmol/l 5-aminosalicylic acid to N-acetyl-5-aminosalicylic acid. 4. N-Acetyltransferase activity was detected in the cytosol, with a mean of 3.3 nmol min-1 mg-1 of protein. There was no N-acetyltransferase activity in the brush border. There was no difference in enzyme activity between epithelial cells derived from normal, Crohn's disease or ulcerative colitis patients. 5. Preliminary characterization of the N-acetyltransferase enzyme suggests a molecular weight of 150,000.