Male mice were injected with 211At and the total absorbed dose to the whole blood, kidneys, testes and thyroid was determined. Extrapolation to man shows a close agreement with the predictions of the International Commission on Radiological Protection (ICRP) for the absorbed dose to human blood, kidneys and testes, but the absorbed dose to the thyroid is about 200 times greater than the ICRP prediction. Comparison of the reduction in testes mass and in sperm numbers 28 days after mice were injected with 211At, or exposed to x rays, indicates a factor of about four for the greater effectiveness of 211At alpha particles over 250 kVcp x rays for the induction of effects.