Purified vaccinia virus was irradiated by 7-rays under direct-effect conditions. The ability of the irradiated samples to form plaques (infectivity) and to induce viral DNA synthesis was determined. The radiosensitive volume of the viral unit causing infection (I.9 × IO-X7 cm2) is very small compared with the volume of the whole viral DNA (~ IO %). The inactivation of the D N A replication function follows a simple exponential law. The radiosensitive volume necessary for the replication of DNA

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... 1-6 × IO -is cm. a) represents only 8"5 % of the DNA necessary for infectivity and o'85 % of the total viral DNA. This indicates existence of a dissociation between two functions of vaccinia virus, the synthesis of viral DNA and infectivity.