Serotherapy, an approach currently used to protect humans against animal bites or stings, is often too specific. To broaden antiserum paraspecificity, use of antibodies directed against areas shared by all members of a toxin family was previously proposed. MST2 is a mAb that recognizes all long-chain curaremimetic toxins (Charpentier et al. (1990) J. Mol. Recog. 3, 74-81). It binds to toxin residues that make contact with the toxin's target, e.g., the nicotinic acetylcholine receptor (AcChoR). We now show that MST2 also recognizes (-) nicotine, an agonist of AcChoR. Binding properties of MST2 therefore mimick, at least partially, binding properties of AcChoR. Injection in rabbits of MST2 mixed with adjuvant, elicited anti-idiotypic (anti-Id) antibodies that inhibited binding of the toxin to AcChoR. A proportion of these anti-Id antibodies specifically bound AcChoR and thereby mimicked the toxin. Furthermore, rabbits immunized with MST2 elicited auto-anti-anti-Id antibodies capable of binding the toxin. Our data provide a molecular explanation for the previously reported signs of myasthenia gravis as triggered by antibodies raised against cholinergic antagonists. Implications in the design of antisera to toxic proteins are discussed.