The mouse CD1 (mCD1) is a class I-like molecule that is encoded outside the MHC. Recent studies demonstrate that mCD1 presents hydrophobic peptides to CD8+ T cells and also that it is recognized by a population of NK1.1+ T cells that are thought to play an immunoregulatory role because of their ability to secrete IL-4. It has previously been reported that mCD1 is expressed predominantly by intestinal epithelial cells, although most NK1.1+ T cells are located elsewhere. We, therefore, have generated new mAbs to mCD1 to investigate its tissue distribution. The principal site of mCD1 expression in normal mice is on cells in the hemopoietic series, including constitutive expression on nearly all T and B cells, on macrophages, and on dendritic cells. Other than bone marrow-derived cells, mCD1 is not widely expressed and is not detectable on great majority of intestinal epithelial cells. The B cells, but not the T cells, from beta2m-deficient mice can be recognized by two mCD1 autoreactive T hybridomas. Therefore, although we could not detect a beta2m-independent form of mCD1 using these mAbs, mCD1 in a different conformation or a mCD1-related molecule is likely to be expressed in the absence of beta2m on some cell types. The pattern of expression of mCD1 correlates with the distribution of NK1 T cells and is consistent with an important Ag-presenting function for this molecule.