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The determination of 90Sr in seawater, sediment and biota is carried out by radiochemical analysis. The choice of method is dependent on the amount of sample to be analyzed, the Ca/Sr mass ratio and the natural Sr content of the sample. For large volumes of seawater and sediment samples (e.g., coral) of high Ca content, 1 g (minimum) of Sr carrier and 85Sr tracer are used. The Sr fraction is separated and purified chiefly by Sr(NO3)2 precipitations. After 2 - 3 weeks, the ingrown 90Y is separated from the parent 90Sr, and the 90Y beta activity is measured by a gas-flow proportional counter. The detection limits obtained are 36 µBq/L for seawater and 0.36 Bq/kg for corals. For sediment and biota samples of low to moderate Ca content and low natural Sr content, 10 - 20 mg of stable Sr carrier are used without 85Sr. The Sr fraction is separated and purified using crown ether extraction chromatography. The purified Sr fraction itself (containing 90Sr together with in-growing 90Y) is measured immediately using liquid scintillation counting. The detection limit obtained is 0.66 Bq/kg for 10g of sediment or biota ash