Photosystem II particles from spinach and barley contained 2.5 and 4.2 Cu per 300 chlorophylls respectively. This Cu was resistant to removal by EDTA. A large percentage of the PSII Cu in both plants is associated with the light-harvesting chlorophyll a/b protein, LHCII; 46% in barley and 76% in spinach. Several experiments have been performed to rule out the possibility that the Cu was introduced during the isolation procedures and to ensure that the Cu is associated with PSII. Since the PSII Cu is mainly associated with LHCII, it is unlikely that it is involved in O2 evolution.