Selected parts of individual Chinese hamster cells were irradiated with alpha particles from a polonium-tipped microneedle; the cells were observed for up to 14 days, and the results compared with those of 200-kV x-radiation. Large doses of alpha-radiation (>25,000 rads) delivered to the cytoplasm had no effect on proliferation; nuclear alpha-irradiation killed cells via giant or multinucleate formation. Hence the targets for cell killing by alpha-radiation are either nuclear or in the immediately perinuclear cytoplasm; most of the cytoplasm is insensitive. Reasons are given for believing that this result can be extended to x-rays. Small doses of alpha-radiation produced a delayed decrease in growth rate in surviving cells. The effects from x-radiation were similar qualitatively to those in conventional Puck cloning experiments; after 270 R, killed cells, small colonies, and survivors were seen, and after 1,000 R most cells divided at least once but later degenerated.

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