IN 1960, Nowell1 first described a plant lectin from Phaseolus vulgaris—phytohaemagglutinin, which possessed mitogenic activity for human peripheral blood lymphocytes, that is, the ability to transform the resting lymphocyte into blast like cells. Subsequently, a second phytomitogen was reported2 in saline extracts of the plant Phytolacca americana—the pokeweed mitogen. In recent studies in our laboratory3,4, we identified and isolated pokewood mitogen in an electrophoretically homogeneous form by preparative multiphase zone electrophoresis on polyacrylamide gel columns and we have shown that it possesses haemagglutinating, leukagglutinating and mitogenic activity. Further chemical characterization5 revealed that pokeweed mitogen is a glycoprotein with a molecular weight of 32,000 containing 3.2 per cent monosaccharide, 1.4 per cent hexosamine and a unique amino-acid composition with thirty-three cystine residues. In addition, biochemical, physiochemical and immunochemical studies revealed that pokeweed mitogen differed from phytohaemagglutinin in a number of important ways4,5.