Intraepithelial lymphocytes have been examined histochemically in the small intestines of normal rats and mice and in thymus-deficient animals (B rats and nude mice). It is concluded that these cells are heterogeneous, consisting of at least two subpopulations. One population contains granules, is thymus-independent, and is probably bone-marrow-derived. The other population does not contain granules and appears to be thymus-dependent. It is suggested that the granular cell, which may be a precursor of mucosal mast cells, is more properly a previously unrecognized nonlymphoid leukocyte of bone marrow origin.