Monoclonal antibodies raised against purified acetylcholine receptor from muscle and electric organ were tested for cross-reaction with surface components on chicken ciliary ganglion neurons. Indirect immunofluorescence indicated that antibodies to a determinant in the "main immunogenic region" of the receptor bind to the neurons in culture. Ultrastructural studies on 16-day embryonic ganglia, using horseradish peroxidase-conjugated monoclonal antibody, revealed that most of the conjugate labeling was associated with synaptic membrane on the neurons. A lesser amount of labeling was associated with the short processes extending from the neuronal somata in the region of preganglionic innervation. The labeling was blocked by coincubation with unlabeled antibodies of appropriate specificity and not by nonimmune serum. The pattern of labeling was clearly different from that previously found for a horseradish peroxidase conjugate of a-bungarotoxin: the toxin conjugate bound extensively to the short processes but not to synaptic membrane on the neurons. The synaptic antigen identified here by the crossreacting antibodies is a candidate for the synaptic acetylcholine receptor on chicken ciliary ganglion neurons.Chicken ciliary ganglion neurons have nicotinic acetylcholine (AcCho) receptors that mediate chemical synaptic transmission through the ganglion (1, 2). Efforts to study the regulation and distribution of such receptors on neurons have been limited by the absence of suitable probes. a-Bungarotoxin has been a very useful probe for studying nicotinic AcCho receptors in vertebrate skeletal muscle and electric organ, where the toxin binds tightly and specifically to the receptor and blocks its function (3). Ciliary ganglion neurons also have high-affinity binding sites for a-bungarotoxin, but in this case the binding sites appear to be distinct from synaptic AcCho receptors. Ultrastructural studies demonstrate that toxin binding sites on the neurons are located in the immediate vicinity of presynaptic terminals but are not present in the postsynaptic membrane (4), as would be expected for synaptic receptors. Moreover, a-bungarotoxin does not block AcCho receptor function on the neurons (5, 6), and, when the levels of AcCho sensitivity and toxin binding are compared for the neurons grown under various conditions in cell culture, no simple correlation is found between the two properties (7), as would be expected if they represented the same membrane component.An alternative approach is to use antibodies against the AcCho receptor as probes. Monoclonal antibodies (mAbs) against AcCho receptors from skeletal muscle and electric organ have been useful in studying the structure and synthesis of these receptors (8)(9)(10)(11)(12). Recent studies have shown that mAbs specific for each of the four subunits of AcCho receptors from muscle and electric organ bind to neurons in the lateral spiriform nucleus of chicken brain (13). These neurons do not bind a-bungarotoxin. Best cross-reaction was obtained with mAbs to the "...