The villous component of nine colonic polyps (three pure villous adenoma and six mixed adenomatous and villous adenomas) was studied by light and electron microscopy. Normal colonic mucosa was used for control. The predominant cell was an undifferentiated epithelial cell resembling those of the base of the crypts of normal colonic mucosa. A minority of these cells differentiated toward goblet cells and absorptive cells. Interdigitations of the plasma membrane with enlarged intercellular spaces were frequently observed, possibly related to the loss of water and electrolytes in some of these tumors, suggesting an analogy to renal tubular plasma membrane changes in kalliopenic nephropathy. Numerous membrane-bound, inter and intracellular "inclusion bodies" representing degenerated lymphocytes and epithelial cell organelles were commonly found. Viral inclusions were not detected in this material. The possibility that the inclusion bodies observed in villous adenoma may represent lysosome-modified vesicles, secondary to absorption of protein and surface mucosubstances appears unlikely.