Humoral proteins from Amphioxus lanceolatus showed a considerable agglutinating activity for human red cells. The haemagglutinating factor eluted in the macroglobulin peak on Sephadex G200 and resulted ConA-1 at affinity chromatography on Sepharose ConA. Immunoelectrophoresis, carried out using a specific antiserum produced in rabbit, revealed alpha mobility for the agglutinating factor, thus suggesting that it was an alpha macroglobulin.