This paper is concerned with the induction of experimental allergic asthma by active sensitization of the airways "vis-à-vis" heterologous protein. Thus in the rodent it is possible to sensitize the airways by direct intra-tracheal injection of the allergen without producing an allergic alveolitis; the concomitant injection of an adjuvant is however indispensable. Confirmation of the sensitized state can be made by in vitro measurements of isometric contraction of tracheal smooth muscle in the presence of the allergen. This active sensitization is accompanied by histocytological changes which are detected by immunomorphological techniques: the rise in the number of plasma cells sensitized to IgE in the mediastinal modes: the appearance (or increase in number) of intra-cytoplasmic IgE in the sub-epithelial tracheal mast cells and in certain non ciliated intra-epithelial cells, probably basophils, morphologically close to the mast cells.