CD9 is a cell surface protein implicated in intercellular signaling that has been identified in selected cell types of the hematopoietic system. To begin a study of the role of CD9 in the developing and adult nervous system, we used the anti-rat CD9 monoclonal antibody ROCA2 to determine the distribution of this protein. The identity of the antigen in these tissues was confirmed by immunoblotting and peptide sequencing. Early embryonic sympathetic and dorsal root ganglion sensory neurons and adrenal chromaffin cells all express CD9. ROCA2 also labels the somas, axons, and growth cones of cultured sympathetic and sensory neurons. In the central nervous system (CNS), CD9 is transiently and specifically expressed in embryonic spinal motorneurons. In the adult, central and peripheral glia intensely express CD9. Thus, CD9 is developmentlly regulated in a variety of peripheral and central neurons and glia, including proliferating progenitors as well as mature cells. These findings suggest that CD9 may have diverse roles in the nervous system. © 1993 Wiley-Liss, Inc.