Cultured cells of a human neuroblastoma, SK-N-MC, were found to be highly resistant to Sabin attenuated poliovirus types 1 and 2 strains; no appreciable cytopathic effect was observed, and the total harvest was generally in the order of 1 PFU per cell or less. On the other hand, related neurovirulent strains of these antigenic types produced a relatively good (2 orders of magnitude higher) yield in a markedly protracted infectious cycle. The limited growth of the attenuated virus in the neuroblastoma cells appeared to be confined to a minor cell subpopulation. Experiments with intratypic (type 1) poliovirus recombinants suggested that the major genetic determinants limiting reproduction of the attenuated polioviruses in the neuroblastoma cells are located in the 5' half of the viral RNA, although the 3' half also appears to contribute somewhat to this phenotype. The possibility that neuroblastoma cells may represent an in vitro model for studying poliovirus neurovirulence is briefly discussed.

Similar physicochemical characteristics were found with strains of Crimean hemorrhagic fever (CHF) and Congo viruses. The particle size, sedimentation coefficient, buoyant density, weight of the particles as well as morphology and morphogenesis of these viruses were similar to those of other members of the Bunyaviridae family. Data are presented on reproduction of CHF virus in cell cultures and on the inner structure of its virion.

Morphological characteristics of Crimean Hemorrhagic Fever Virus indicate that it is closely related to the Bunyaviridae family. However, there are some features (e.g. formation of crystal-like patterns in infected cells) demonstrating differences to the other known Bunyaviridae.

SUMMARYA study was made of the mechanism of microfocus formation and of the nature of the inclusion bodies that appear in the cytoplasm of RK-I3 cells infected by rubella virus.Direct passage of virus particles from one cell to another, at least in the early stages of the infection, is posed as a possibility, and the lysosomal nature of the cytoplasmic inclusion bodies is demonstrated by histochemical methods. A description is given of conditions required in the monolayer for the formation of a microfocus, which is the characteristic manifestation of the c.p.e, of the rubella virus in this cellular system.

In the present report some general physicochemical properties and RNA characteristics of strain 258 of enterovirus (EV) 71, isolated in 1975 from cases of poliomyelitis-like disease in Bulgaria (1), are described. It is concluded, that the features of the virus are consistent with the previous identification, based on biological studies, of this virus strain as a member of enterovirus subgroup of picornaviridae family.

It is shown that reovirus capsid inner layer consists of 132 capsomeres with a skewed icosahedral pattern of T = 13 (left). 780 structural units composing capsomeres are integrated into trimers. The boundary between capsomeres runs between the structural units forming trimers.

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