The lectin from Datura stramonium purified by affinity chromatography on Sepharose-fetuin was able to agglutinate lymphocytes as well as erythrocytes. The agglutination of both cell types was specifically inhibited by oligomers of N-acetylglucosamine. The lectin preparation also possessed mitogenic activity which could be inhibited specifically by N-acetylgucosamine oligomers. The mitogenic activity, but not the agglutinating (lectin) activity, was libile to incubation at 65°C for 30 min. It is suggested that a single lectin can agglutinate both red and white blood cells, but is distinct from a mitogen of similar saccharide specificity.