SUMMARY: Interference was demonstrated with pantropic Rift Valley fever virus in appropriately ultraviolet-irradiated mouse serum in which a residuum of active virus remained. The degree of interference varied with the amount of irradiation to which the virus had been exposed. With the smallest doses of ultraviolet radiation, which left a fair amount of active virus, the only manifestation of interference was a prolongation of the incubation period. With virus which had received optimal irradiation, concentrated inocula elicited no symptoms but the same material in higher dilution produces the typical fatal illness. With doses of ultraviolet radiation between the minimum and the optimum, virus suspensions were obtained which showed interference, but many of the mice developed signs of neurological involvement after a prolonged incubation period. Although the symptoms elicited were neurological in origin, the virus recovered from the brain was unaltered pantropic variety. The distribution of this virus in the affected mice was determined. It is suggested that the residual live virus contained in irradiated material undergoes a growth cycle in a limited number of liver and other susceptible cells not protected by the interference effect of inactivated virus. The late neurological manifestations are attributed to this newly formed virus and the lesser affinity of the brain cells for the interfering inactivated virus. Many of the mice which survived inoculations of concentrated irradiated virus were shown to be immune to subsequent infection with 100 MLD of Rift Valley fever virus. This immunity is considered to be due to a latent infection initiated by residual live virus present in the irradiated material. Virus subjected to prolonged irradiation probably loses its immunizing power because of complete inactivation rather than destruction of the antigenicity of individual virus particles.


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