Ricin A chain has been isolated from toxic RCA II. Our ricin A chain has less than 0.01% of the toxicity of the native lectin in a cell culture test system, yet is as potent as native ricin in a cell-free protein synthesis assay. The A chain can be linked to other proteins such as antibodies. These antibody-hybrid molecules can be used to kill specific cells with the appropriate antigenic determinant exposed, such as tumor markers on surfaces of malignant cells. The A chain is supplied in buffered saline with sodium azide as a preservative.