An X-ray fluorescence method for the analysis of biological samples of any thickness has been developed. Concentration values are calculated from the characteristic X-ray line intensities of Zgte15 atomic number elements using only one reference standard for calibration. For the excitation of elements ⁵⁵Fe and ¹²⁵I annular radioisotope sources are used. Measurements have been carried out with a conventional X-ray spectrometer on various types of IAEA, NBS and NIES standard biological samples. Good agreement has been obtained between certified and measured concentration values.