Phenotypic and functional analysis was performed with lamina propria mononuclear cells (LPMCs) isolated from colonoscopic biopsies in 27 patients with ulcerative colitis (UC). The proportion of T lymphocytes displaying HLA-DR antigens, interleukin 2 (IL-2) receptor, and transferrin receptor was greater in active UC than in control diseases. When LPMCs were cultured with IL-2 or phytohemagglutinin for 72 h, there were no significant differences in the proportion of cells bearing these activation markers between active UC and controls. The proportion of CD56+ cells and lymphokine-activated killer (LAK) cell activity was lower in LPMCs from active UC than in control cells, and depletion of CD56+ cells from control lamina propria cells essentially eliminated LAK cell activity. Mucosal T lymphocytes may be activated in vivo during active inflammation in UC, and lower levels of intestinal LAK cell activity may be related to the decrease of CD56+ cells under these conditions.