SummaryNumerous forms of a pleomorphic microorganism were observed associated with the cytoplasm of epithelial cells in conjunctival smears from sheep with pink-eye when stained by the Giemsa method. The microorganism was coccobacillary in shape during the acute stage of infection, and stained purplish blue. Various peripheral forms, and dispersed, crescent-, horseshoe-, and ring-shaped figures were present in post-acute smears; all these pleomorphic forms stained blue. Peripheral forms were filaments, blebs, stalked and club-shaped figures, and ring forms. Intracytoplasmic red spherical bodies were the characteristic entity in the epithelial cells of "normal"t sheep smears. The developmental forms of the pink-eye and trachoma agents were morphologically different.Acridine orange staining of representative forms of the pink-eye agent indicated the presence of ribonuleic acid only, in contrast to the demonstration of ribonucleic and deoxyribonucleic acids which appear in sequence during the developmental cycle of the trachoma agent. The iodine method stained the glycogen matrix of trachoma inclusions a brown colour, but a similar stain re/l>Ction was not visible in pink-eye or normal sheep smears.Changes in the cytology of pink-eye smears suggested a gradual replacement of neutrophils successively by large, medium, and small lymphocytes as infection progressed; sometimes plasma cells were present.Mycopla8ma agents were isolated from conjunctival scrapings of sheep with pink-eye. A transmission experiment with one of the isolates produced a mild pinkeye condition. Coccobacillary forms were evident in smears from sheep which developed pink-eye; eye smears from control sheep were normal.It is postulated that the aetiological agents of pink-eye in sheep are Mycoplasma spp. The significance of this finding is discussed in this paper and the syndrome is described.

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