The mycoplasma-like gray lung and rat pneumonia agents were indistinguishable in fine structure and location in infected mouse lung. Both agents resembled mycoplasmas in fine structure, being bounded by a 110 A unit membrane which enclosed an internal region of 20 to 30 A branching fibrils and groups of 120 A ribosomes. They differed from mycoplasmas in having an 80 to 90 A electron-lucent space separating the unit membrane from the internal material, and the internal region was bounded by a 30 to 40 A unbranched fibril. In division, the organisms appeared to form a transverse trilayered “division plate.” In overall shape, the organisms seemed to be thick circular or oval plates, and they tended to lie in palisade arrangement on cell surfaces. Both agents were free in large numbers in the alveoli and respiratory bronchioles, and on the surface of intro-alveolar mononuclear cells, but were not seemingly associated with lung cells as such. Intracellular organisms were within membrane-bounded inclusions
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