The present study is concerned with a hemagglutinin extracted from the seeds of the osage orange (Maclura pomifera Raf.). No previous reports of agglutinins from this plant could be found. The aims of this investigation were to characterize the serologically active component from Maclura pomifera to as great an extent as possible, to determine if the substance has any specificity under varying environmental conditions, to identify its receptor, and to gain some idea of the natural role of the material in the plant.