DNA encoding ricin B chain was derived from preproricin genomic DNA and ligated into the baculovirus transfer vector, pAcGP67A. Co-transfection of Spodoptera frugiperda Sf9 cells with BaculoGold DNA was followed by limiting dilution purification of recombinant baculovirus. Infection of SF9 cells at a multiplicity of infection of 1 in the presence of 25 mM lactose produced 3 mg/l of soluble, glycosylated, 34 kDa protein immunoreactive with monoclonal and polyclonal antibodies to ricin B chain. The recombinant ricin B chain had similar lectin-binding activity to plant ricin B chain. The recombinant protein reassociated with ricin toxin A chain similarly to ricin toxin B chain and the recombinant heterodimers had similar cell cytotoxicity to ricin.