Copper resistance in Pseudomonas syringae carrying the copABCD operon is associated with accumulation of copper in the periplasm and outer membrane, apparently as a function of the copper-binding activities of the copABC gene products. However, no specific function for copD has been determined. In this study, P. syringae cells containing copCD or copBCD cloned behind the lac promoter were hypersensitive to copper. An increased accumulation of copper was measured in cells containing several combinations of cop genes that included copC and copD. Our data suggest that CopC, a periplasmic copper-binding protein, and CopD, a probable inner membrane protein, may function together in copper uptake.