ABSTRACTAn extract fromJatropha curcasseeds, purified by gel filtration on Sephadex G‐75 and Sephacryl S‐200, yielded an active hemagglutinin of high purity as assessed by electrophoresis and isoelectric focussing. The hemagglutinin had a molecular weight of around 660,000 and a pI value of 5.75. The molecule was composed of two different subunits of molecular weights 23,450 and 11,500. Amino acid analysis suggested that the molecule lacked 1/2 cystine but contained a high proportion of acidic and basic amino acids. Agglutination of trypsinized erythrocytes, groups A, B and O, took place over the range pH 4–10, and was prevented byD‐galactose,D‐galactosamine andN‐acetyl‐D‐galactosamine. The hemagglutinin has only a weak binding capacity forD‐galactose. Its activity was stable up to 60°C; at 80°C activity