The following study cinfirms previously reported electron-microscopic findings in ulcerative colitis and agrees with the nonspecific nature of those findings. The study extends these observations in regard to relationship of PMN leukocytes, eosinophil leukocytes, and mast cells to colonic epithelial cells. Strong, though not conclusive, data that PMN and eosinophil leukocytes extensively invade epithelial cells in ulcerative colitis is presented. This finding also is not specific to ulcerative colitis and was found with much less frequency in Crohn's disease of the colon and in salmonella colitis. The presence of intravascular degranulation of PMN leukocytes in ulcerative colitis is confirmed. This study adds additional support to the concept that the major abnormality in ulcerative colitis resides within the colonic epithelial cell.