The use of tritium-labeled thymidine (3 H- TdR) in biological research made it necessary to develop a quick and accurate method for determination of tritium activity in tissue. After3 H- TdR incorporation into the root tip meristem of Vicia faba, total3 H activity as well as <tex-math>${}^{3}{\rm H}\text{-}{\rm DNA}$</tex-math> activity was measured by liquid scintillation counting. The incorporation rate of3 H- TdR using various parameters was examined-for example, the amount of incorporated3 H- TdR as a function of duration of treatment or as a function of thymidine concentration in the nutrient solution. The experimental results together with other data allow the calculation of the average number of incorporated thymidine molecules per labeled cell nucleus. This is necessary to interpret quantitatively the biological effects of incorporated radionuclides.

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