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SummaryMorphological changes developing in BI{K-21 cells infected with the S/N virus (a recombinant of Ap/Singapore and A/NWS viruses), at the approximate m. o. i. of 1 TCDs0 per cell, were studied by electron microscopy. Substantial alterations were observed in both the nucleus and cytoplasm. Changes in the nuclei were first detected eight hours after infection. The nucleoli substance was reduced and dense inclusions consisting of fibrillar structures 40--60 • in diameter were formed. The greatest number of inclusions and their distribution over the whole nuclear area were found 24 hours after infection. Electron-dense inclusions were also detected in the cytoplasm. They were identical in shape and size with those observed in the nuclei but they consisted of fibrils 35--40 A in diameter. These inclusions were rare eight hours after infection. Their number increased with the incubation period, reaching a maximum 24 hours after infection. Six hours after infection filamentous structures 80--90 A in diameter were observed in the cytoplasm. It is assumed that they represent viral ribonueleoprotein. They were at first (hour 6) distributed diffusely through the whole cytoplasm and were later (hour 8--10) found mainly near the cytoplasmic membrane. Twenty four hours after the infection the filamentous structures were abundant both in the central part of the cytoplasm and in the proximity of the cellular membrane. Sometimes they formed dense agglomerations 2000--4200 A in width. Naturation of the virion took place at the plasma membrane and in cytoplasmic vacuoles in whose proximity the filamentous structures were found. The particles either budded directly from the smooth cell membranes or from processes formed on the cell surface. Numerous rod-like particles were observed.

SUMMARYThe reproduction of a mutant of A2/srNoAPORE/57, selected after repeated passages in monkey kidney cells, and the synthesis of its subviral components were followed in two systems: in monkey kidney cells, in which new infectious virus is formed, and in human diploid cells, in which the growth cycle of this virus is abortive. The formation of subviral antigcns was investigated by both immunofluorcscence and complement-fixation tests. No marked differences were found between the kinetics of synthesis of the V and S antigens and their total yields in both systems. Migration of the S antigen from the nuclei was observed in A2/SlNOAPORE virus infected by human diploid cells.

SUMMARYThe replication of influenza viruses A/NWS-D, A/WS-MK and their rl~ recombinant in human embryo fibroblast (HEF) and human diploid fibroblast (LEP) cell lines was studied. In HEF cells virus NWS-D and recombinant r12 induced synthesis of virus-specific macromolecules and produced infectious virions; virus WS-MK induced synthesis of virus complementary RNA (cRNA), virion RNA (vRNA), proteins, RNP and non-infectious virions, but haemagglutinin cleavage was impaired and the virions formed contained uncleaved haemagglutinin. In LEP cells, infectious virions were formed only by virus NWS-D; viruses WS-MK and r~2 induced synthesis of virus cRNA, vRNA, proteins and RNP; virus rlz had the haemagglutinin cleaved, whereas in virus WS-MK this process was impaired; neither virus WS-MK nor r~2 was capable of forming virions.

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