Cluster of differentiation 1 (CD1) in humans is a family of major histocompatibility complex (MHC) class I-like molecules expressed on the surface of immature thymocytes, Langerhans cells, and a subpopulation of B cells. The only function identified for human CD1 is as a ligand recognized by a subpopulation of T lymphocytes. In order to study the distribution and function of these molecules in the mouse, a murine CD1 complementary DNA was expressed in mouse fibroblasts and used to produce monoclonal antibodies. These antibodies revealed prominent expression of murine CD1 only on gastrointestinal tract epithelium and in the cytoplasm of hepatocytes. Low levels of expression were also detected on thymocytes and peripheral lymphocytes. The gastrointestinal distribution of murine CD1 suggests that this molecule may be important in epithelial immunity.