The CD1 antigens are a family of differentiation antigens found predominantly, but not exclusively, in the human thymus. Although three antigens (CD1a-c) are described by monoclonal antibodies, five genes (CD1A-E) are found in the human genome. The cloning of the mouse CD1 genes (Bradbury, A., Belt, K.T., Nery, T.M., Milstein, C. and Calabi, F., EMBO J. 1988. 7:3081) demonstrated the presence of homologues to human CD1D, but not to any of the other human CD1 genes. In this work we have examined the expression of mouse CD1D mRNA in the thymus and shown that it is predominantly cortical, as is the expression of the CD1 antigens in man. Somewhat surprisingly, we also find that most CD1D mRNA in the mouse thymus is unspliced. Despite this, we have also been able to show, using a polyclonal antiserum directed against a bacterial fusion protein, the existence of the expected protein product.