MULTIPLICITY reactivation was first demonstrated in 1947 by Luria1, in bacteriophages irradiated by ultraviolet light. In 1951, Henle and Liu2 showed the same phenomenon in influenza viruses irradiated with ultraviolet light and grown in chick embryos. Their results were confirmed by Barry3 in 1961. In the same year Polley4, investigating the sterilizing potentialities of ionizing radiations, found that viral infectivity could be destroyed completely by high doses of cobalt-60 gamma rays while viral haemagglutinin (HA) appeared more resistant. In these experiments, the loss of infectivity was a perfectly exponential function of the radiation dose, and no evidence of multiplicity reactivation was observed.