Protein kinase C (PKC) activity was measured in the inflamed colonic mucosa of 24 patients with ulcerative colitis and in the normal colonic mucosa of 10 patients with other benign diseases. The particulate fraction activity in ulcerative colitis mucosa was significantly increased compared with that of normal mucosa (320 +/- 47 versus 200 +/- 30 pmol/min/mg protein; p less than 0.05). Inflamed ulcerative colitis mucosa also showed significantly increased total PKC activity in the particulate fractions compared with normal mucosa (147 +/- 26 versus 37 +/- 8 pmol/min/mg tissue; p less than 0.05). Mucosal samples from ulcerative colitis patients were divided into 12 with mild and 12 with severe inflammation by histologic examination. The particulate PKC activity of severely inflamed mucosa was significantly lower than that of mildly inflamed mucosa (p less than 0.05). These results indicate that colonic inflammation in ulcerative colitis may be associated with altered cellular PKC activity.