A new lectin from soybeans having specificity toward the extracellular 4-O-methyl-D-glucurono-L-rhamnans produced by certain strains of Rhizobium japonicum has been purified and characterized. Isolation was accomplished initially by isoelectric precipitation of contaminating globulins and subsequently by affinity chromatography on partially hydrolyzed glucuronorhamnan covalently coupled to amino-hexylagarose. Residual globulins were removed by adsorption of the lectin on concanavalin A-agarose and elution with methyl alpha-mannoside. The lectin is a glycoprotein (3-5% carbohydrate) with a molecular weight of approximately 175 000. It is a tetramer with subunit molecular weights of 45 000 when dissociated with sodium dodecyl sulfate. Reverse-phase high-pressure liquid chromatography analysis indicates the presence of two types of subunits, both having equivalent molecular weights. According to amino acid analyses, the lectin is rich in acidic but low in sulfur-containing amino acids. The carbohydrate portion of the lectin contains mannose; no hexosamines could be detected. Chemical modification of the lectin indicated that neither sulfhydryl groups nor amino groups participate in binding. Quantitative binding studies of the lectin with various carbohydrate haptens showed that specificity was directed toward 4-O-methyl-D-glucuronic acid, D-glucuronic acid, and their methyl glycosides with 4-O-methyl-D-glucuronic acid 3-4-fold more effective. In each instance, the methyl glycoside is a more effective hapten.