A study was made of the replication of Riley virus in various tissue cultures. It was observed that Riley virus replicated in primary mouse embryo cultures for 8 to 12 days. As they aged primary cultures became less susceptible to Riley virus infection, and subcultured cells were not susceptible. Suspensions of virus, in the absence of cells, were inactivated at 36.5°C. Serial passage by transfer of supernatant fluids to fresh embryo cultures was not successful. Replication of the virus was more active in mouse embryo liver cultures than in the cultures of the embryo minus the livers.

In cultures of mouse macrophages, the supernatants remained infective throughout the life of the cultures (21 days). The virus was passed 23 times through fresh macrophage cultures over a period of 80 days, during which the original inoculum (106.0ID50/ml) was theoretically diluted to 10–14.

The possibility that the cells of the RES are involved in Riley virus replication is discussed.

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