Since the etiological relation of the pollens of certain plants to hay fever and some forms of asthma was established, a number of investigators have studied the question of the nature of the active substances in the pollens, as well as the mechanism of the curative or alleviating effect of the two forms of specific treatment (with “antitoxin”—“pollantin” of Dunbar—and with active immunization as first practised by Noon). Originally, both of these methods of treatment were based upon the assumption that the active substances of the pollens are toxins (toxalbumins) to which the former method induced a passive immunization, while the latter, as already indicated, caused an active immunization by the production of specific antibodies. This conception has been supplanted by the theory of Wolff-Eisner (1), 1 according to which the clinical manifestations of hay fever are expressions of hypersensitiveness to an otherwise non-toxic constituent of the pollens.