A highly active form of wheat germ agglutinin (WGA) was isolated by affinity chromatography on a partially acid hydrolyzed chitin column after extraction of the wheat germ with 0.5 M formic acid and removal of the denatured or water insoluble WGA by dialyzing against distilled water before and after affinity chromatography. The purified preparation was found to be homogeneous by gel filtration, disc electrophoresis, and chemical analysis. It reacted readily with WGA receptors in human serum and urine, giving well-defined bands on agar gel double diffusion and electrophoresis. When chemically coupled to Sepharose the WGA was very reactive with red blood cells, WGA receptors in serum, urine and other biological fluids. The Sepharose-WGA has proven to be stable over a long period of time.