Enterococcus hirae grow well under anaerobic conditions at alkaline pH (pH 8.0) producing acids by glucose fermentation. Bacterial growth was shown to be accompanied by decrease of redox potential from positive values (approximately +35 mV) to negative ones (approximately -220 mV). An oxidizer copper (II) ions (Cu(2+)) affected bacterial growth in a concentration-dependent manner (within the range of 0.05 mM to 1 mM) increasing lag phase duration and decreasing specific growth rate. These effects were observed with the wild-type strain ATCC9790 and the atpD mutant strain MS116 (with absent beta subunit of F(1) of the F(o)F(1) ATPase) both. Also ATPase activity and proton-potassium ions exchange were assessed with and without N,N'-dicyclohexylcarbodiimide (DCCD), inhibitor of the F(o)F(1) ATPase. In both cases (DCCD +/-), even low Cu(2+) concentrations had noticeable effect on ATPase activity, but with less visible concentration-dependent manner. Changes in the number of accessible SH-groups were observed with E. hirae ATCC9790 and MS116 membrane vesicles. In both strains Cu(2+) markedly decreased the number of SH-groups in the presence of K(+) ions. The addition of ATP increased the amount of accessible SH-groups in ATCC9790 and decreased this number in MS116; Cu(2+) blocked ATP-installed increase in SH-groups number in ATCC9790. H(+)-K(+)-exchange of bacteria was markedly inhibited by Cu(2+), but stronger effects were detected together with DCCD. Moreover, discrimination between Cu(2+) and other bivalent cation--Ni(2+) was shown. It is suggested that Cu(2+) ions inhibit E. hirae cell growth by direct affect on the F(o)F(1) ATPase leading to conformational changes in this protein complex and decrease in its activity.