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Rabbits 6 weeks old, given single I.V. injections of z3@Pu as Pu(NO,), were killed at different times after injection. Undecalcified sections of femurs and vertebrae were prepared, and the dose rates determined by track counting on autoradiographs. I n trabecular bone, 24 hours after injection, the ratio of maximum to minimum dose rates t o sten1 cells outside the endosteal surface is about 3 : 1. All the trabecular bone surfaces hare a deposit of plutonium. At 16 weeks after injection, about one fifth of the trabecular hone surface still has a significant dose rate, and only a few per cent of the surface has no plutonium deposit, even though nearly all the trabecular bone has been formed since injection. Sincc most of the endosteal bone surface is trabecular bone, a large volume of stem cells and marrow cells is irradiated h p these deposits. There is also a significant dose rate due to plutonium in the marrow.

Summary Adult human lymphocytes from five individuals were exposed to ultrasonic irradiation at power levels ranging from 0.014 to 2.77 W/cm.2, and at different stages of the cell cycle. No excess of chromosomal damage was detected in such cells as compared with unirradiated control samples.

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