A low molecular weight non-precipitating antigen from timothy pollen extract, Antigen G (AgD), is monovalent for the major antigen which induces reaginic antibody in humans. Antigen G inhibits specific reagin activity in vitro and appears to suppress its formation when administered in alginate to allergic patients. An understanding of molecular composition of AgD, i.e. hapten and carrier determinants, and of how it suppresses at a cellular level would provide insight into how reagin production is regulated, and would also provide a basis for defining and isolating suppressive fractions from other allergens. As a consequence, therapy of allergic disease might be improved.