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Commercially obtained phytohemagglutinin (PHAP) derived from Phaseolus vulgaris contains 17 different protein bands when analyzed by acrylamide gel electrophoresis. When it is subjected to CM-Sephadex chromatography followed by molecular sieving on Sephadex G150, several species of potent mitogenic proteins, which differ greatly in their hemagglutinating capacity, are obtained. A low hemagglutinating mitogen (L-PHAP), homogeneous by several different criteria, is the most potent mitogen isolated, and also possesses potent leukoagglutinating activity. It is a glycoprotein with a molecular weight of 115,000, containing glucosamine, mannose, xylose, and fucose or arabinose. Also isolated is a mixture of at least two closely related proteins possessing high hemagglutinating capacity, with hemagglutination titers 250 times more potent than L-PHAP. This material is a slightly less potent mitogen than L-PHAP and also possesses leukoagglutinating capacity, although of a lower order of magnitude. Its amino acid and carbohydrate composition are similar to L-PHAP, but it contains approximately twice as much carbohydrate and is slightly larger as determined by molecular sieving.