The activation of H+-ATPase solubilized from plasma membrane of rice (Oryza sativa L. var Nipponbare) culture cells was examined by the exogenous addition of various phospholipids, free fatty acids, glycerides, polar head groups of phospholipids and molecular species of phosphatidylcholine (PC). H+-ATPase activity appeared to be stimulated by phospholipids in the following order: asolectin > phosphatidylserine > PC > lysophosphatidylcholine > phosphatidylglycerol, and maximal ATPase activation was noted at around 0.05 to 0.03% (w/v) of asolectin or molecular species of PC. Polar head groups such as glycerol, inositol, and serine only slightly activated ATPase activity or not at all, while ethanolamine and choline had no effect. Activation was dependent on the degree of saturation or unsaturation of the fatty acyl chain and its length. The plasma membrane of plant cells contains a H+-ATPase which produces a proton electrochemical gradient across the plasma membrane, thereby providing the driving force for transporting nutrients into cells (19,20). Acidification stimulated by auxin may also cause the loosening required for cell enlargement, suggesting that the plasma membrane H+-ATPase is involved in auxin action (13). The mechanism for regulation of the ATPase by auxin is unknown (17). In our previous paper (10) and other reports (2,3,18), plasma membrane H+-ATPase was shown to require phospholipids for its activation, indicating that its activity may be regulated by interactions between this protein and the phospholipid environment in the plasma membrane. However, the effects