Evidence has been presented that influenza viruses both of type A and B partially inactivated by ultraviolet irradiation may regain their capacity to propagate in the allantoic membrane of the chick embryo. In using such irradiated preparations as inocula for growth curve experiments it could be shown that the development of hemagglutinins as well as of infectivity preceded at rates resembling those noted with more than 10 times the amount of infective virus actually found in the irradiated seed. Partial inactivation of the inocula by heating to 56°C. gave similar results.

The phenomenon was observed only with seed irradiated for short periods of time so that the virus particles sustained only few hits of radiation. On prolonged exposure resulting in numerous hits per virus particle the capacity of reactivation was lost. Likewise, an irradiated preparation capable of reactivation in the allantoic membrane, could not be diluted more than about 30-fold and still clearly produce this phenomenon. This indicated that reactivation is obtained only when one host cell adsorbs more than one non-infective virus particle but not upon adsorption of a single particle.

These data are in striking agreement with the phenomenon of "multiplicity reactivation" observed in the bacteriophage-E. coli system by Luria and Dulbecco.

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