Dry phage T1 was irradiated with monochromatic uv radiation (150-254 nm) in vacuo. The inactivation sensitivity was highest at 150 nm. On Escherichia coli${\rm B}_{{\rm s}-1}$, the phage inactivation sensitivity was two and four times higher at 150 nm than at 254 and 230 nm, respectively. Action spectra for phage inactivation on both E. coli B and${\rm B}_{{\rm s}-1}$ fit the absorption spectrum of phage DNA, except around 190 nm. The host-cell-reactivable fraction for vacuum-uv radiation (below 190 nm) was smaller than that with far-uv radiation. There was almost no photoreactivation at 150 nm, in contrast to a photoreactivation sector of about 0.3 at 254 nm.

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