Bungarus multicinctus venom was fractionated by ion exchange chromatography and the various fractions were assayed for their ability to block synaptic transmission through the chick ciliary ganglion. alpha-Bungarotoxin purified from this venom failed to block transmission at 50 micrograms/ml. A second neurotoxin, which we designate Toxin F, blocked transmission at 1-3 micrograms/ml and also blocked ganglionic depolarizations induced by carbachol. Toxin F was clearly distinguishable from alpha-bungarotoxin on the basis of molecular weight (estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis) and isoelectric point. Binding assays revealed that 125I-labeled toxin F bound to two sites in the ciliary ganglion: one site that was shared by alpha-bungarotoxin and toxin F and another site that was recognized solely by toxin F. Carbachol and d-tubocurarine displaced only that [125I]toxin F bound to the shared site and had no effect on [125I]toxin F bound to the site recognized by toxin F alone. The results suggest that toxin F blocks synaptic transmission in the chick ciliary ganglion by a postsynaptic mechanism. Further study is required to determine whether this effect of toxin F is mediated through a direct interaction with ganglionic nicotinic receptors.