The lungs of South African sheep, in a district in which jagziekte is appearing sporadically, differ from those of normal American sheep. Their structure is in general variable. They are more subject to the action of bacteria, demonstrable histologically, and to the effect of inhaled foreign material. The most significant difference, however, centers in the interalveolar tissue, which, in about 33 per cent of the animals, is definitely thickened beyond the range of variation in this direction observed in American sheep. The thickenings occur in localized areas several millimeters in diameter. They begin with engorgement of the alveolar capillaries and accumulation of macrophages and of lymphocytes. Many of the macrophages pass into the alveolar lumina and assume the appearance of typical epithelioid cells. These infiltrative and exudative changes are primary to the epithelial proliferations which always arise in tissues modified in this way and which it is proposed to describe in a second paper.