Cu2+ ion determinations were carried out in complex and in inorganic salts-glycerol media, to which increasing amounts of Cu(II) had been added, with the ion-specific Cu(II)-Selectrode. Likewise, complexing capacity of bacterial suspensions was estimated by titration with CuSO4. Copper-sensitive bacteria, e.g., Klebsiella aerogenes, were inhibited in their growth and survival in the range of 10-8-10-6 M Cu2+ ion concentrations. In copper-buffered complex media, high copper loads could be tolerated, as growth proceeded with most of the copper bound to medium components. In low-complexing mineral salts media, in which high Cu2+ ion concentrations exist at low copper loads, there was competition of Cu2+ for binding sites of the cells. Total allowed copper was then determined by the ratio of copper to biomass. Copper-resistant bacteria could be isolated from a stock solution of CuSO4, containing 100 ppm Cu(II). They were of the Pseudomonas type and showed a much higher tolerance towards Cu2+, up to 10-3 M.