Properties of the anion ATPase from rat red blood cell membranes were investigated. Diethyl ether treated membranes exhibited the increased activity of the anion ATPase. Na+, K+- and Ca2+-ATPase activities were not found in these preparations. The pH optimum of the anion ATPase was at pH 8.5. The enzyme was stimulated by methanol and inhibited by glycerol. Among the inorganic anions stimulators, inhibitors and indifferent substances were observed. Anions of thiocyanate, sulfite and bicarbonate altered noncompetitively the ATPase activity. Sulfite stimulated and thiocyanate inhibited the ATP hydrolysis in presence of magnesium, calcium, zinc, cobalt, manganese and nickel. Reactions with ATP, ITP, GTP but not with ADP and AMP used as substrates were sensitive to sulfite and thiocyanate. EGTA did not change the stimulation and inhibition effects of the anions on the ATPase activity. The similarity of properties of erythrocyte and mitochondrial ATPases is discussed.