RESUMEN
Affinity-purified antibodies to the nicotinic acetylcholine receptor of Torpedo marmorata were fractionated into two populations using a covalently cross-linked receptor-toxin immunosorbent lacking free toxin-binding sites. The population of antibodies which bound to and were subsequently eluted from this resin, and which cannot possibly contain antibodies directed to the toxin-binding site itself, was effective in inhibiting the binding of toxin to receptor in solution. This unequivocally demonstrates that inhibition of toxin binding can be mediated by antibodies which are not directed against the toxin-binding site. A second minor population of antibodies which did not bind to the affinity resin but which did inhibit the binding of toxin to receptor in solution was detected. Two subpopulations of toxin-binding inhibitory antibodies can therefore be distinguished. A clear differentiation should be made in future work describing "anti-toxin site" antibodies between antibodies directly binding to the toxin-binding site and the pseudo-anti-toxin-binding site antibodies described in this report.