The use of 67Cu-labeled antibodies for the treatment of cancer has advanced to the clinical trial phase. Quantitation of 67Cu radiopharmaceuticals is complicated by the presence of the radioimpurity of 64Cu in 67Cu supplies. Here we report a method to assay 67Cu and 64Cu in a mixed sample with a commonly available instrument, the ionization chamber dose calibrator. Methods: The activities of 67Cu and 64Cu in a mixed sample can be calculated from a single-dose calibrator measurement. The calculation requires (1) instrument-specific response coefficients D67 and D64, generated by gauging the instrument for the efficiency of measurement of 67Cu and 64Cu, and (2) a value for the ratio of 67Cu to 64Cu in the sample, routinely provided by major suppliers of 67Cu. D67 and D64 were empirically determined by measuring samples containing known amounts of 67Cu and 64Cu. The samples were also assayed by gamma ray spectroscopy to verity the isotope ratios given by the suppliers. Results: This method generated accurate response coefficients. At the recommended dose calibrator setting for the measurement of 67Cu, at which D67 = 1.0, the measurement for D67 with this method was 1.02 (± 0.04). Isotope ratios provided by the radionuclide suppliers were corroborated by gamma ray spectroscopy. Conclusion: A method is presented by which 67Cu and 64Cu in a mixed sample can be assayed using a dose calibrator. Although the derived numeric constants are only correct for a specific dose calibrator and setting, the method can be adapted for use with any dose calibrator.