In this paper, the authors present a sensitive method for low level non-exchangeable OBT determination by a combination of a suitable sample treatment, a combustion apparatus for large sized samples and low background liquid scintillation spectrometry, along with a methodology that eliminates any possibility of sample contaminations. Great care must be taken in the measurement of non-exchangeable OBT at environment levels, because many authors have mentioned the opportunities for cross contaminations between samples and contamination by exchange with the laboratory atmosphere. Our analysis are reproducible for low level OBT down to about 1 Bq.kg" 1 of dry material for various biological samples (tree wood, tree leaves, fishes, sediments, mushrooms, seaweeds, wheats, corns, ...) collected in the general environment and near nuclear facilities. As an example, we give the results of non-exchangeable OBT measurements in annual growth rings from two trees sampled near a nuclear facility.