We have isolated mouse cDNA clones encoding Tyro 3, a receptor protein-tyrosine kinase (PTK) of the mammalin central nervous system (CNS). Expression of the Tyro 3 gene is strongly up-regulated in neurons of the mouse neocortex, cerebellum, and hippocampus after the day of birth, during periods of active synaptogenesis, and high expression is maintained in the adult CNS. The sequence of Tyro 3 cDNAs predicts a glycoprotein receptor with similarity to neural cell recognition and adhesion molecules--the extracellular (ligand binding) region of this receptor is composed of two immunoglobulin-related domains followed by two fibronectin type III repeats. Immunoblot and immunoprecipitation analyses with anti-Tyro 3 antibodies indicate that the 125 kD Tyro 3 protein is abundantly expressed in CNS synaptosomes, and immunohistochemical analysis of cultured hippocampal cells demonstrates that Tyro 3 is a product of neurons. Rat-2 fibroblasts stably transfected with a Tyro 3 expression construct acquire the ability to grow in soft agar, suggesting that Tyro 3 is potentially oncogenic.