Specific binding of monoclonal antibodies RAC 17, 18, and 23 to ricin A chain. Ricin holotoxin (1.25 μg/lane) was electrophoresed on a 10% SDS polyacrylamide gel, then stained for protein with a Commassie stain (column 2) or blotted onto nitrocellulose for subsequent exposure to a 1:10,000 dilution of the indicated hybridoma immunoglobulin. Each immunoglobulin bound the 32,000 Dalton A subunit of ricin, but was minimally reactive with the 34,000 Dalton B subunit (columns 3–5). Ctl IgG is a control mouse monoclonal IgG also used at 1:10,000 dilution (column 6). MW standards are shown in column 1. Lower arrow, 32,000 Dalton subunit; upper arrow, 34,000 Dalton subunit.