The L1 family comprises transmembrane cell adhesion molecules of the immunoglobulin superfamily that play an important role in neuronal migration and axon outgrowth, fasciculation, and myelination. Consistent with a crucial role in developmental processes, mutations in L1 cause severe brain malformations. Although L1 activates intracellular signaling pathways, little is known about the membrane proximal events of L1 signaling. The cytoplasmic domains of L1 family proteins contain several conserved tyrosine residues that are potential targets for receptor tyrosine kinases. Here, we report that the L1 family protein Ng-CAM is phosphorylated on tyrosine in embryonic day 13 chicken retina. This is the first demonstration of in vivo tyrosine phosphorylation of an L1-like molecule. Because chicken embryo kinase 5 (Cek5) is a receptor tyrosine kinase expressed in neuronal processes and activated in the chicken embryonic retina, we have analyzed the possible role of Cek5 in L1 phosphorylation. The rat glioblastoma cell line B28 was stably transfected with human L1. Additional transient transfection with Cek5 cDNA led to expression of Cek5 in its tyrosine-phosphorylated, activated form. Biochemical analysis revealed that L1 is phosphorylated on tyrosine in Cek5-transfected cells but not in control transfectants. Furthermore, direct phosphorylation of the L1 cytoplasmic domain by Cek5 was demonstrated in an in vitro kinase assay. Tyrosine phosphorylation may represent a novel mechanism of signal cascade initiation through L1.