Electron microscopic observation was made on microfold cells (M cells) in the covering epithelium of the lymphoid follicle (dome epithelium) of the intestine. Materials consisted of ten human appendices, five of those were inflamed and obtained from children with acute appendicitis. The remainder was not inflamed macroscopically, and there was one human Peyer's patch for control. The results indicate that in the human appendix, the elevated surface type of M cells named by protruding apical cytoplasm to the lumen was more conspicuous than the depressed surface type. The latter type was named by shorter irregular microvilli than those of neighboring cells, and was present dominantly in human Peyer's patch. M cells with enfolded lymphocytes consisted of the stumpy type and the slim type in the whole shape. M cells in the inflamed appendix showed their apical cytoplasm swelling like a balloon and microfolds disappearing, and seemed vulnerable to inflammation. It is considered that the M cell surface structure changes not only in accordance with enfolded lymphocytes and the uptake of antigenic materials, but also according to the organ in which M cells are present and whether inflammation is present or not.