[en] Generally it is said that trace radiation measurement is achieved by increasing the counting efficiency and the amount of sample and decreasing the background counting rate. It often occurs that a detector does not operate or counting efficiency decreases considerably in internal source method, in which sample is sealed in the detector, when various conditions are not set properly. Thus the first problem is how to prepare the sample of such chemical form that does not decrease the counting efficiency. The second problem is to lower the background counting rate as far as possible, and it necessitates to adopt procedures such as coincident counting and the selection of detector and shielding materials. In the practical measurement of ³H, most examples employ emulsified or hydrophilic scintillator, while some examples adopt benzene synthesis or isotope exchange method. In the above practice, minimum measurable tritium concentration is ca. 100 p Ci/l if a water sample is measured without receiving any treatment. Isotopic enrichment is required for the samples that have less concentration than the above. (Wakatsuki, Y.)