Amber mutants of bacteriophage <tex-math>${\rm T}_{4}{\rm D}$</tex-math> were used to prepare phage head and tail components. The isolated heads and tails were purified and their response in aqueous suspension to ionizing radiation measured. The radiosensitivities were compared with that of whole bacteriophage. Both the heads and tails were inactivated exponentially. The Do (37% survival) of the tails was 1.6 krad compared with 1.2 krad for whole phage and 5.0 krad for heads. In nutrient broth the DNA containing heads were similar but slightly more sensitive to radiation than whole phage, while the protein tails were considerably more resistant. In dilute suspension however the tails were similar but slightly more resistant to radiation than whole phage but the heads were substantially more resistant than tails or whole phage. The major lethal target for phage irradiated in dilute aqueous solution is probably located in the tail.

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