Abstract 1. 1. Isotopic copper exchange in Pila leopoldvillensis haemocyanin was studied with 64Cu(II) acetate and with three 64Cu(I) complexes, including a separation of excess copper with Chelex-100 chelating resin. 2. 2. With 64Cu(II)acetate and with 64Cu(I)-acetonitrile complex the radioactive copper incorporation into oxygenated or deoxygenated haemocyanin was found to be very small. 3. 3. With 64Cu(II) acetate in the presence of KCN at the utmost 41% of the copper content of haemocyanin was replaced by the radioactive isotope. The experiments with K 3 64Cu(CN) 4 at different concentrations indicated that a reversible isotopic exchange reaction occurred with only half of the protein copper. Deoxygenation of haemocyanin had no influence upon the rate and extent of copper exchange. The copper band decreased linearly as a function of copper exchange and almost disappeared when the copper exchange approached 50%. 4. 4. Likewise with 64CuCl 2 −, in the absence of O 2, only half of the protein copper is exchangeable. The rate of exchange with 64CuCl 2 − is, however, much faster than with cyanocuprate.