Escherichia coli cells transformed with plasmids containing ricin B-chain coding sequences are shown to express this heterologous protein in inclusion bodies. After denaturation and renaturation of the product in the presence of glutathione and lactose, the recombinant ricin B-chain is soluble, biologically active and stable. Cytotoxicity of heterodimer containing this protein and ricin A-chain is found to be only ten times lower, than that of native ricin. Recombinant B-chain alone was nontoxic to cells (ID50 > 10(-6) M). Our data suggest that ricin B-chain oligosaccharides are essential for stability preserving protein from proteolytic degradation in cells.