The determination of Ra-228 by means of gamma-spectrometry, in material containing significant quantities of K-40 and Ca2+ such as bone ash results in increased values of counting uncertainty and lower limit of detection (LLD) because of a significant contribution from the Compton continuum of K-40. However, K-40 is widely removed from bone ash if Ra-228 is coprecipitated with barium sulfate. As a result, the counting uncertainty and LLD are significantly reduced. A method is presented for determination of very low activity concentrations of Ra-228. Impurities introduced by precipitation are negligible when applying high resolution gamma-spectrometry.