Chinese hamsters, of mixed sexes and 90-110 days old, were injected intraperitoneally with 0.45, 0.15, 0.05, 0.017, 0.006, 0.002 and 0.0 μCi of <tex-math>${}^{144}{\rm Ce}-{}^{144}{\rm Pr}/{\rm g}$</tex-math> body wt and partially hepatectomized at 4, 13, 40, 120, 360 or 720 days postinjection. The animals were injected with colchicine at 50 hr postsurgery, and sacrificed 4 hr later to measure the aberration frequency in the metaphase chromosomes. The dose to the liver was calculated using retention data combined with data on the activity of <tex-math>${}^{144}{\rm Ce}-{}^{144}{\rm Pr}$</tex-math> in the liver at the time of hepatectomy and sacrifice. The frequency of chromosome aberrations increased linearly through 362 days with an aberration coefficient of <tex-math>$3.1\times 10^{-4}$</tex-math> aberrations/cell/rad. For doses up to 14,500 rads through 362 days of the study, there were no signs of saturation or plateauing of the radiation dose effect curve. At 722 days postinjection, the slope of the dose-response curve had decreased to <tex-math>$1.8\times 10^{-4}$</tex-math> aberrations/cell/rad and the aberration frequency in the controls had more than tripled. Changing dose rate over a range of 2-250 rads/day had no effect on the frequency of aberrations/cell/rad. The efficiency of producing aberrations per rad of dose was equal to that observed after protracted60 Co exposure. Over the first 362 days, the dose required to produce a level of chromosome damage that was two standard deviations higher than observed in the controls was 130 rad. There were three hepatocellular neoplasms and one bile duct adenoma observed in animals injected with the three highest levels of activity.

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