The amino acid sequence was determined for toxin F, a component of Bungarus multicinctus venom that blocks nicotinic transmission in the chick ciliary ganglion and the rat superior cervical ganglion. Toxin F was purified by a procedure that includes preparative isoelectric focusing and ion exchange chromatography. Seventy nanomolar toxin F blocks nicotinic transmission in the chick ciliary ganglion; however, the toxin only weakly blocks the binding of 125I-alpha-bungarotoxin to membranes derived from Torpedo californica electroplax (IC50 = 1 microM). These data raise the possibility that toxin F may preferentially recognize neuronal nicotinic receptors. Toxin F focused identically on an isoelectric focusing gel with samples of two similar toxins, bungarotoxin 3.1 and kappa-bungarotoxin. The sequence of toxin F is identical with that recently reported for kappa-bungarotoxin. When the N-terminal portion of bungarotoxin 3.1 was sequenced, it was found to be identical to the other two toxins. These and other data suggest that the 3 toxins are, in fact, the same.