Summary The nature of the protein responsible for the hemagglutinating activity of Datura metel extract has been investigated. Ammonium sulfate precipitation and paper electrophoresis indicate that the active protein is a globulin similar to human γ-globulin. A globulin, having an electrophoretic mobility similar to that of human γ-globulin, was shown to be present in the extract. The active protein was not digested by trypsin or papain, unlike other proteins in the seed extract. A simple procedure is proposed for the purification of the active protein.