Functional significance of CD1 has long been enigmatic since it was defined as human thymus differentiation antigens first by polyclonal rabbit antibodies and then by clusters of monoclonal bodies. 1 Recently, several reports showed that CD1 worked as antigen presenting molecules for lipid antigens and as a target ligand for unusual T cells such as human γδ T cells, double negative αβ T cells and mouse natural killer T cells. 2–4 Because there are considerable differences in their characteristics between mouse and humans, it is not yet clear such conclusions could be generally applied in both species. To solve the problem we investigated the structure, expression and function of rat CD1. 5