A highly accurate and precise procedure deserving the name of definitive method has been devised for the determination of copper in biological materials. The method is based on combination of neutron activation and very selective and quantitative post-irradiation separation of copper from other radionuclides by extraction chromatography, using columns with LIX 70 on Bio-Beads SM-1, followed by gamma-ray spectrometric measurement. All potential sources of errors were carefully examined and eliminated or appropriate corrections were introduced into the procedure. The method contains several warning mechanisms safeguarding against making gross errors. Limit of detection for rather short irradiation time (tir1 h) (which enables radiochemical work without sophisticated shielding) amounts to 15 g/kg. Results for copper concentration in several NBS, IAEA and other certified reference materials are presented and a new recommended value for IAEA's Milk Powder (A-11) is proposed.