Proteins of the immunoglobulin superfamily (IgSF) are involved in a variety of specific cell-cell interactions in the developing nervous system. We used a systematic approach, based on the PCR, to identify and characterize new members of this protein family in the developing chick nervous system. In this study, we report on CEPU-1, a new IgSF protein that is strongly expressed in developing Purkinje neurons in the cerebellum and is detectable on the dendrites, somata, and axons of these cells. The timing of CEPU-1 expression coincides with the growth of the dendritic tree--after the Purkinje cells have finished their migration from the ventricular zone. CEPU-1 is a 51 kDa glycoprotein comprised of three Ig-like domains of the C2 subcategory and is anchored to the plasma membrane via a post-translationally attached glycosyl-phosphatidylinositol moiety. The protein shows high sequence similarity to the neurotrimin glycoprotein (78% identity), to the opioid-binding cell adhesion molecule (or OBCAM; 68% identity), and to the limbic system-associated membrane protein (or LAMP; 54% identity). Our results suggest that CEPU-1 may serve as a cellular identification marker that is specific for developing Purkinje cells.