HCO3--sensitive ATPase was found in nuclear and plasma membrane fractions of Ehrlich ascites tumour cells and lymphoma NK cells. HCO3--ATPase was not sensitive to monovalent cations and to ouabain (10(-4) M). The 60 mM HCO3- is the concentration of maximal activation of the HCO3--sensitive ATPase. The HCO3--sensitive ATPase was inhibited by anions in the sequence: SCN- greater than F- greater than ClO4- greater J-. The anions Br-, NO3-, HSO3- were not effective.