A comparative study of the respiratory syneytial (RS) virus and the pneumonia virus of mice (PVM) discloses very few dissimilar details of structure, morphogenesis and biological characteristics. Peak titer of the I~S virus on tIEp-2 cells at an input multiplicity of 5:1 was obtained between 24 and 72 hours whereas that of PVM on Vero cells at an input multiplicity of 1.5:1 was reached between 48 and 96 hours. The replication of PVM is followed by a definite but still incomplete cytopathic effect at 11 days while that of I~S virus results in a complete cell degeneration after 3 days. These two viruses show the same kind of eosinophilic cytoplasmic inclusion bodies with hematoxylin-eosin. These inclusions are stained yellow-green with acridine orange. Indirect immunofluorescenee reveals the presence of viral antigens only in the cytoplasm of the cells, detectable as early as 8 hours in the case of RS virus and at 24 hours in the case of PVM. By negative staining, the diameter of the RS virus spherical particle is 100 to 350 nm and that of PVM 100 to 200 nm. In addition to these spherical forms, pleomorphic and filamentous forms are frequently encountered, the latter predominating in PV1V[ preparations. The diameter of both RS virus and PVM nucleoeapsids averages 13.5 nm and the pitch of the helix is 6.5 nm compared to 17.5 nm and 5.0 nm for parainfluenza type 2 used as an internal control. The RS virus and PVM envelopes are studded with projections of 12 nm in length, 10 nm apart in the ease of RS virus but only 6 nm apart in the ease of PVM. In ultrathin sections, RS virus and PV3/[ appear to bud from the cytoplasmic membrane mainly into filamentous 1 This investigation was supported by the MedieM Research Council of Canada grant MA4179. These results were presented in part at the Annum Meeting of the

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Degradation of purified rubella virus by heat treatment (37, 45, or 56 degrees C) revealed the following structures. The viral envelope, a modified cellular membrane, bears spherical subunits, 5-6 nm in diameter, hexamers, or pentamers. Two glycoproteins, VP-2 (50 000 daltons) and VP-3 (63 000 daltons), are associated with the envelope. The nucleocapsid if formed by the condensation of the viral ribonucleic acid on acentral structure 10 nm in diameter. Only one protein, VP-1 (35 000 daltons) is present in the nucleocapsid. Similarity between rubella virus and Togaviruses is discussed.

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