Recent reports highlight the impact of copper on lactate metabolism: CHO cell cultures with higher initial copper levels shift to net lactate consumption and yield lower final lactate and higher titers. These studies investigated the effects of copper on metabolite and transcript profiles, but did not measure in detail the dependences of cell culture performance and product quality on copper concentrations. To more thoroughly map these dependences, we explored the effects of various copper treatments on four recombinant CHO cell lines. In the first cell line, when extracellular copper remained above the limit of detection (LOD), cultures shifted to net lactate consumption and yielded comparable performances irrespective of the differences in copper levels; when extracellular copper dropped below LOD (approximate to 13 nM), cultures failed to shift to net lactate consumption, and yielded significantly lower product titers. Across the four cell lines, the ability to grow and consume lactate seemed to depend on the presence of a minimum level of copper, beyond which there were no further gains in culture performance. Although this minimum cellular copper requirement could not be directly quantified, we estimated its probable range for the first cell line by applying several assumptions. Even when different copper concentrations did not affect cell culture performance, they affected product quality profiles: higher initial copper concentrations increased the basic variants in the recombinant IgG1 products. Therefore, in optimizing chemically defined media, it is important to select a copper concentration that is adequate and achieves desired product quality attributes. (c) 2014 American Institute of Chemical Engineers Biotechnol. Prog., 31:226-238, 2015