A/B toxins, produced by bacteria and plants, are among the deadliest molecules known. The B chain binds the cell, whereas the A chain exerts the toxic effect. Both anti-A chain and anti-B chain Abs can neutralize toxins in vivo and in vitro. B chain Abs block binding of the toxin to the cell. It is not known how anti-A chain Abs function. Working with ricin toxin, we demonstrate that immunization with A chain induces greater protection than immunization with B chain. A panel of mAbs, binding to A chain, B chain, or both chains, has been produced and characterized. Immunologic characteristics evaluated include isotype, relative avidity, and epitope specificity. The ability to inhibit ricin enzymatic or cell binding activity was studied, as was the ability to block ricin-mediated cellular cytotoxicity on human and murine cell lines. Finally, the in vivo protective efficacy of the Abs in mice was studied. The Ab providing the greatest in vivo protective efficacy was directed against the A chain. It had the greatest relative avidity and the greatest ability to block enzymatic function and neutralize cytotoxicity. Interestingly, we also obtained an anti-A chain Ab that bound with high avidity, blocked enzymatic activity, did not neutralize cytotoxicity, and actually enhanced the in vivo toxicity of ricin. Anti-A chain Abs with moderate avidity had no in vivo effect, nor did any anti-B chain Abs.