1. A method is described for the differential radioactivity counting of 125I and 3H in a liquid-scintillation spectrometer without a separate gamma counter.

2. The sample was contained in a polyethylene miniature vial placed centrally in a standard 20 ml glass scintillation vial containing a tin-loaded scintillant.

3. A direct measure of the 125I radioactivity at an efficiency of 30% was then obtained by radioactivity counting in the pre-set 3H window of a scintillation spectrometer. No counts for 3H radioactivity were registered at this stage because of the barrier to the passage of the low-energy β-particles provided by the wall of the polyethylene vial.

4. After mixing the sample and scintillant both 125I and 3H were detected at efficiencies of 73 % and 29% respectively. Subtraction of the 125I contribution from the combined radioactivity count rate then gave the net 3H count.

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