The lymphoid cells in the epithelium covering the villi of the small intestinal epithelium of the mouse were examined by light, fluorescence and electron microscopy. The population has a heterogeneous morphology with small and medium-sized lymphocytes predominating. Large lymphocytes and various types of pyroninophils are less common. The fate of these cells is also diverse, a small proportion degenerates, some migrate back into the lamina propria, and the remainder are probably lost into the lumen of the intestine. The presence of blast cells, blast cell-lymphocyte interaction, and limited antibody production suggests that an active immunological reaction is taking place in the epithelium. Whether the large number of non-pyroninophils in the epithelium are also involved in this reaction, or whether they have a different immunological function is not known.