Freeze-dried preparations of the scrapie agent and of 5 strains of virus were used to determine the extent of protection afforded against ionizing radiation by anoxia. The five viruses tested were bacteriophages T1 and T2 (DNA) and µ2 (RNA); and the animal viruses herpes simplex (DNA) and yellow fever (RNA). Oxygen enhancement ratios (measured in terms of doses required to give the same effect in the absence and presence of oxygen) varied from about 1.4 for the scrapie agent to 2.5 for yellow fever virus. With the viruses there was no obvious correlation of oxygen enhancement ratio with type or size of nucleic acid core.

‘Target sizes’ of the viruses were calculated from the doses required to give an average of one lethal event per particle. These agreed well with independent estimates of size of nucleic acid core; except that for bacteriophages T1 and T3 the calculated molecular weight was 0.3 of that determined by Bresler (1967). Results with the scrapie agent confirmed the previous estimate (Alper, Haig & Clarke, 1966) of a molecular weight of about 1.5 × 10 daltons.


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