We wanted to identify and characterize MAbs with specificity for the toxic lectin ricin, which could serve as detection reagents in elucidating mechanisms and tissue distribution. Neutralizing MAbs could be developed into immunotherapeutics to reverse clinical intoxications from immunotoxin or to counteract the use of ricin as a terrorist or biological warfare weapon. Two hybridomas, UNIVAX 70 and 138, producing MAbs against ricin were identified by Western blot strip analysis. The antibodies were IgG1 and were specific for the ricin A chain with no ricin B chain cross-reactivity. The MAbs neutralized ricin in vitro in an EL-4 mouse leukemia cell assay and in an in vivo mouse model. The two antibodies recognized the same epitope or overlapping epitopes, based on a competition with one another. All further characterization proceeded on the assumption that they were the same. The MAb UNIVAX 70/138 was characterized in vivo by titrating it against an 18 micrograms/kg (> six LD50) i.v. challenge and by titrating the i.v. toxin challenge against a constant dose of 100 micrograms of passive antibody per mouse. A 4:1 molar ratio of MAb to ricin led to neutralization of > or = 90% of the toxin in vitro. The MAb recognized ricin toxoid prepared by formaldehyde treatment and after conjugation of low molecular weight haptens (based on ELISA) equally as well as it recognized ricin and ricin A chain. The affinity and specificity of UNIVAX 70/138 give it excellent reagent potential, and the toxin-neutralizing capacity makes it at least a log and a half better than the next best candidate immunotherapeutic.