Affinity-purified phytohemagglutinin from red kidney bean resolves into five isolectins by SP-Sephadex ion exchange chromatography. Recoveries ranging from 30 to 130 mg of protein for each isolectin are easily achieved. The isolectins have similar amino acid compositions which differ only in threonine, lysine, and arginine. A distinguishing feature of the amino acid composition is the total lack of sulfur-containing amino acids. Each isolectin contains about 4% mannose and 2.2% N-acetyl-D-glucosamine. All isolectins on electrophoresis form single protein bands under denaturing and nondenaturing conditions in polyacrylamide gels, and all have apparent subunit molecular weights of 33,000 by sodium dodecyl sulfate polyacrylamide gel electrophoresis. The isolectins are also homogeneous by ultracentrifugation and have apparent native molecular weights of 115,000 +/- 4,130, suggesting tetrameric quaternary structures. Whereas 80% of the starting erythroagglutinin activity is recovered, one of the five isolectins possesses 50% of that original activity. As sequentially eluted from the ion exchange column, each isolectin displays progressively higher erythroagglutinin and lower lymphocyte mitogenic activities. Based on their relative biological activities, the isolectins are assigned the structures L4, L3E1, L2E2, L1E3, and E4, where L and E represent lymphocyte- and erythrocyte-reactive subunits, respectively, and the subscripts represent the proposed subunit composition.