A simple experimental method for the efficiency calibration of germanium detectors especially for environmental samples is presented, using only the natural radionuclides in the sample. The method is based on the fact that for the energy range above 300 keV the full-energy-peak efficiency of a Ge detector can be described in a first order approximation by a linear interpolation curve in the log-log display with errors lying normally under 5%. Photons with different energies which are emitted from one radionuclide yield count rates which are correlated to the corresponding efficiencies. From this correlation one coefficient of the interpolation curve — a first order polynomial — can be calculated. The second coefficient can be obtained by the count rate of40K, resulting from KCl, which is mixed homogeneously with the sample. Expecially for environmental samples with large volumes, this method is very useful, because it takes into account the self-absorption of photons in the sample.