WHEN radioisotopes are used in clinical or biological studies, there is often a risk that the radiation from the isotope may produce disturbances in the system studied. This risk may be negligible in acute experiments but is not always so in long-term studies, especially when large doses of isotopes are used or when one type of tissue specifically coa5entrates the tracer.The clearance of intravenously injected colloidal particles from the circulating blood has been used by many workers as a test of phagocytic activity or of liver blood flow. Since the quantities of solid matter invo1vc.d in such a test are preferably very small ( 10-5-10-8 g/sample in our experiments) the use of artificial isotopes as tracers has meant an enormous increase in t'he scope and accuracy of such work.The most usual technique then for investigation of blood clearance is intravenous injection of a radioactive colloid, and measurement of the activity in serial blood samples. The circulation half-life of the colloid is usually taken as an expression of blood clearance capacity. Investigations of this t'ype have been carried out in human and animal studies using colloidal 198Au (
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