Norleucine methylester was coupled to carboxylates of fasciculin 2, a snake toxin that inhibits acetylcholinesterase (AChE). This neutralized negative charges but had no effect on the activity, suggesting that carboxyls do not participate in binding to AChE. Earlier results are discussed. Modification of three aromatic amino acids in the peripheral site of AChE, the binding site for fasciculin, decreased the affinity 100 to one million times. Neutralizing the charge of cationic groups of fasciculin lowered the affinity only three to seven times. A change in either the toxin or enzyme part of a binding site should have about the same effect. Since this was not so, it suggests that cationic groups of fasciculin do not bind to aromatic rings in the peripheral site.