On the supposition that excessive fibrinolysis at the rectal mucosal level may contribute to the pathogenesis of ulcerative colitis, 11 patients with this condition, in whom rectal bleeding was the predominant feature, were given a course of epsilon-aminocaproic acid therapy. Six patients responded dramatically to this treatment, there was a partial response in two, no effect in two others, and one patient found it necessary to discontinue the treatment after 48 hours because of the severity of side effects.