[en] Full text: Natural radioactivity in bottled mineral water is of particular interest in radiation protection since the consumption of these waters increases in Germany and a considerable contribution to the radiation exposure can result thereby. Because of their radiotoxicity the Ra-226 (member of the U-238 decay series, alpha emitter) and the Ra-228 (member of the Th-232 decay series, beta emitter) are of primary concern. The Federal Office for Radiation Protection (BfS) has performed an investigation programme for the mineral waters produced in Germany. The aim of the study was to ascertain the variation of the radionuclide concentrations and to estimate the resulting radiation exposure to individuals drinking mineral water. Radium nuclides to be analysed were separated from 5 litre samples. Ra-226 was determined by the emanation technique. Both Ra-228 and Ac-227 were determined applying the extraction chromatography. Actinium was isolated using a specific resin for the Lanthanides (Ln-spec). The activity of Ac-228, a short lived daughter product of Ra-228, was measured in a low-background proportional counter for about 16 hours using time gates of one hour. The Ra-226 concentrations ranged from 0.7 to 1690 mBq·l^-1 with a geometric mean of 8.1 mBq·l^-1, whereas the Ra-228 concentrations varied between 4.6 and 930 mBq·l^-1 with a geometric mean of 7.4 mBq·l^-1. The variability of the Ra-226 to Ra-228 concentration ratio (0.13-23) points out that there is no correlation between the concentrations of these two radium isotopes. Consuming mineral water the effective doses for individuals of each age-group are predominantly (80 - 100%) caused by Ra-226 and Ra-228. If the consumption rate for drinking water is assumed for the consumption of mineral water too, the dose values calculated for the critical group (infants) exceed in numerous cases the reference level of 0.1 mSv/a recommended by the World Health Organization (WHO, 1993) for drinking water. (author)