.alpha.-Bungarotoxin and similar neurotoxins from snake venoms are extremely useful for identifying, labeling and quantitating neuromuscular-type nicotinic AChR [acetylcholine receptors]. The specificity of .alpha.-bungarotoxin for AChR appears to be greater than that of d-tubocurarine since, unlike the latter, other nicotinic AChR are not affected. .alpha.-Bungarotoxin has been used to correlate the number of AChR in end-plates with the level of neuromuscular transmission [in rats]. Another group of neurotoxins, exemplified by .beta.-bungarotoxin, crotoxin and taipoxin, also affects neuromuscular transmission, but by a presynaptic mechanism, leaving the AChR unaffected. These agents act by phospholipase A2 action and, at the advanced stage, may cause degeneration of nerve terminals. Since the sites of action of .alpha.- and .beta.-bungarotoxins are so clear-cut, these toxins have been used successfully for the analysis of action of neuromuscular agents, especially of the agonistic type.