A series of cytotoxic neutral dicarboxylatoplatinum(II) complexes containing D(+), L(-) or DL-malate dianion and ethylenediamine or 1-ethylimidazole as ligands were examined using ATPase activity assays and the proton extrusion test. ATPase activity assays in vitro on plasma membrane H+-ATPase and on mitochondrial ATPase were carried out. The concentrations of compounds inhibiting enzyme activity to 50 per cent (J50) was determined. The new platinum complexes showed a stronger level of inhibition of both ATPases than the reference carboplatin; this inhibitory activity is related to a stereoisomeric form of anionic platinum ligands. ATPase inhibition in vivo was tested by glucose-stimulated proton extrusion and the influence of platinum compounds on this process in yeast cells was determined. Significant differences in activity levels were observed between those complexes with 1-ethylimidazole and those with ethylenediamine.