Proteins of the synaptic basal lamina are important in directing the differentiation of motor nerve terminals. One synaptic basal lamina protein, agrin, which influences postsynaptic muscle differentiation, has been suggested to influence nerve terminals as well. To test this hypothesis, we cocultured chick ciliary ganglion neurons with agrin-expressing CHO cells. Ciliary ganglion neurons, but not sensory neurons, adhered five times as well to agrin-expressing cells as to untransfected cells. Further, ciliary ganglion neurites were growth inhibited upon contact with agrin-expressing cells. Finally, the synaptic vesicle protein synaptotagmin became concentrated at contacts between ciliary ganglion neurites and agrin-expressing cells. These activities were shared by neuronal and muscle-derived agrin isoforms, consistent with the hypothesis that muscle agrin may influence the presynaptic axon. Our results suggest that agrin influences the growth and differentiation of motoneurons in vivo.