A lethal protein with hemagglutinating activity but without trypsin inhibitory activity was isolated from beans of Phaseolus vulgaris, cultiva, and Kintoki and proved homogeneous by ultracentrifugation, disc polyacrylamide gel electrophoresis, sodium dodesyl sulfate polyacrylamide gel electrophoresis and isoelectric focusing. The molecular weight was estimated to be 104, 000 by ultracentrifugal analysis and gel filtration on Sephadex G-200. The molecule dissociates into three identical subunits in the presence of 8 M urea or 0.1% sodium dodesyl sulfate. The amino acid composition was characterized by the high content of aspartic acid and the complete absence of methionine and cystine. The carbohydrate content was 8.1%; 5.0% mannose and 3.1% glucosamine. The addition of the lethal protein to a basal diet (0.4%) resulted in the intensive depression of the growth and finally in the death of rats. The intraperitoneal injection of 250 microgram per g body weight of mouse brought about an acute toxicity which caused death of all the injected mice.