The CD1 gene family encodes at least three proteins: CD1a, CD1b, and CD1c, which are coexpressed on cortical thymocytes and a number of T cell leukemias. On thymocytes, CD1a forms noncovalent complexes with CD1b and CD1c, and a disulfide-linked heterodimer with CD8. This report describes the isolation and characterization of cDNA clones encoding the CD1a, CD1b, and CD1c Ag. Cotransfection of the cDNA was used to investigate the formation of intermolecular heterodimers by CD1a with other members of the CD1 gene family and with CD8. No intermolecular heterodimers were observed during transient expression in COS cells. However, an exclusion hierarchy was observed between members of the CD1 gene family when two or more members of the family were cotransfected into COS cells.