The neuronal cell adhesion molecule (CAM) L1 promotes axonal outgrowth, presumably through an interaction with the fibroblast growth factor receptor (FGFR). The present study demonstrates a direct interaction between L1 fibronectin type III (FN3) modules I-V and FGFR1 immunoglobulin (Ig) modules II and III by surface plasmon resonance analysis. Binding of L1 to FGFR1 was enhanced by adenosine 5'-triphosphate (ATP), adenylylmethylenediphosphonate (AMP-PCP), and guanosine-5'-triphosphate (GTP), but not adenosine monophosphate (AMP). The L1-FN3 modules were capable of activating FGFR1, reflected by receptor phosphorylation, and this resulted in the induction of differentiation of primary neurons, reflected by neurite outgrowth. Furthermore, ATP modulated L1-induced neuronal differentiation and FGFR1 phosphorylation through regulation of the L1-FGFR1 interaction.