Peanut seeds contain the globular protein, peanut seed lectin (PSL).  A lectin is a protein of non-immune origin that agglutinates cells and/or precipitates complex carbohydrates.  Lectins are widely distributed in nature and occur in edible legumes, such as kidney beans, soy beans, lima beans, peanuts, and many other plants that are common in human diets.  Peanut lectin is only toxic to people with a peanut allergy and can be used as a surrogate for the highly toxic castor seed lectin, ricin.  The adsorption of purified peanut lectin to clay minerals was examined using batch adsorption isotherms.  X-ray diffraction was used to measure interlayer expansion of clays due to PSL adsorption.  Treatment of 30 mg of Na-montmorillonite with 10 mg of PSL at pH 5 expanded the basal spacing from 1.24 nm to 2.65 nm.  At pH 7, 115 mg PSL/g montmorillonite was adsorbed. The effect of pH on both PSL adsorption and interlayer clay expansion will also be examined.