Pneumonia virus of mice (PVM) particles are spheres 80–120 mµ in diameter, or filaments of similar diameter with lengths up to 3 µ. The particles possess an outer spike-covered envelope and helical internal component 120–150 A in diameter. Virus particles acquire their envelope by a budding process at the cell membrane; mature particles are seen only extracellularly.

Dense inclusions are prominent in the cytoplasm of PVM-infected BHK21 cells by 48 hr after inoculation. The inclusions appear to consist of aggregates of the internal component of PVM, and the helical component has been isolated in a cesium chloride gradient from extracts of osmotically shocked cells.

Murine erythrocytes, which are agglutinated by PVM, adsorb to the surface of infected cells and to budding and extracellular PVM particles.

On the basis of its structure and morphogenesis, PVM appears to be a myxovirus; however, it does not fit into either of the established subgroups of myxoviruses. The 120–150 A diameter of the PVM internal component differs from the diameters of the internal components of the two established subgroups of myxoviruses, and suggests that a third subgroup of these viruses may exist.

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