Abnormalities in humoral immunity are implicated in the pathogenesis of ulcerative colitis. However, the detailed mechanisms of B-cell activation in the locale remain unaccounted for. We analyzed ulcerative colitis from the standpoint of lymphocytic expansion in the loco. Intestinal specimens obtained at surgery from 30 patients with ulcerative colitis treated with corticosteroids and 15 with Crohn's disease were analyzed by immunohistochemistry and flow cytometry. Ulcerative colitis was characterized by a diffuse distribution of Ki-67(+) small round cells particularly in the ulcer base (that were CD19(+) and CD20(-)), with a significant number of them also CD138(+). Immunoelectron microscopy for CD19 revealed an abundance of rough endoplasmic reticulum in the cytoplasm. These indicated that they are of immature plasma lineage cells. By contrast, plasma cells in Crohn's disease were negative for CD19, and the labeling for Ki-67 was infrequent, showing mature phenotype. Flow cytometry revealed an occurrence of CD19(+) and CD20(-) cells in ulcerative colitis but not in Crohn's disease. The labeling index of Ki-67 among CD19(+) plasma cells was positively correlated with the clinical activity of ulcerative colitis. High labeling of Ki-67 in CD19(+) plasma cells is specific for active ulcerative colitis that was resistant to medical treatment by corticosteroids.