A correlated light and electron microscope study was made of lymphocytes in the lamina propria and migrating into the epithelium of the small intestine. C57BL mice and Wistar rats, all of known age and ranging from young adult to senile, were used. The lymphocytes in the lamina propria had a crowded cytoplasm with rough endoplasmic reticulum. A peculiar type of transformation occurred in numerous migrating lymphocytes both on the villi and in the crypts. After they entered the epithelium, the cytoplasm lost most of its endoplasmic reticulum and showed a great decrease in number of mitochondria. The clear lymphocytes then stood out among the epithelial cells. There was also a considerable increase in size of the transforming lymphocytes. Some evidences of integration of these cells with the adjoining epithelial cells were seen, particularly where complex folds of the cells interdigitated. Some of the clear lymphocytes also sent columns of cytoplasm up toward the lumen. An interpretation that the transformed cells stay for some time in the epithelial layer seems reasonable. In the crypts a number of lymphocytes, however, were found in vacuoles within the cytoplasm of epithelial cells and underwent complex degenerative change there.