The artificial insertion of increasing amounts of unsaturated fatty acids into human erythrocyte membranes modulated ATPase activities in a biphasic manner, depending on the number and position of double bonds, their configuration, and the chain length. Uncharged long-chain fatty acid derivatives with double bonds and short-chain fatty acids were ineffective. Stearic acid stimulated Na+ K+-ATPase only. Anionic and non-ionic detergents and alpha-lysophosphatidylcholine failed to stimulate ATPase activities at low, and inhibited them at high concentrations. Mg2+-AtPase activity was maximally enhanced by a factor of 2 in the presence of monoenoic fatty acids; half-maximal stimulation was achieved at a molar ratio of cis(trans)-configurated C18 acids/membrane phospholipid of 0.16 (0.26). Na+K+-ATPase activity was maximally augmented by 20% in the presence of monoenoic C18 fatty acids at 37 degrees C. Half-maximal effects were attained at a molar ratio oleic (elaidic) acid/phospholipid of 0.032 (0.075). Concentrations of free fatty acids which inhibited ATPases activities at 37 degrees C were most stimulatory at reduced temperatures. At 10 degrees C, oleic acid increased Na+K+-ATPase activity fivefold (molar ratio 0.22). Unsaturated fatty acids simulated the effects of calmodulin on Ca2+-ATPase of native erythrocyte membranes (i.e., increase of Vmax from 1.6 to 5 mumol PO43- . phospholipid-1 . hr-1, decrease of K'Ca from 6 microM to 1.4-1.8 microM). Stearic acid decreased K'Ca (2 microM) only, probably due to an increase of negative surface charges. A stimulation of Mg2+-ATPase, Na+K+-ATPase, and Ca2+-ATPase could be achieved by incubation of the membranes with phospholipase A2. An electrostatic segregation of free fatty acids by ATPases with ensuing alterations of surface charge densities and disordering of the hydrophobic environment of the enzymes provides an explanation of the results.