Effects of the elicitor and the suppressor from a pea pathogen, Mycosphaerella pinodes, on the H⁺-translocating activity of ATPase in pea plasma membranes were examined. The plasma membrane ATPase was solubilized with Triton X-100 and partially purified by continuous glycerol density gradient centrifugation. The ATPase fraction was reconstituted into soybean phospholipid (asolectin) liposomes by a Triton-X 100 dilution method. Almost all of resultant proteoliposome vesicles had unimembranes in size from 50 to 200nm. The ATP-driven H⁺-pumping activity was measured by quinacrine fluorescence quenching in the presence of Mg²⁺-ATP. The activity was sensitive to orthovanadate, dicyclohexylcar-bodiimide (DCCD), verapamil and neomycin but it was insensitive to azide and nitrate as well as ATP-hydrolyzing activity. Proton gradient was collapsed by NH₄Cl or a channel-forming ionophore, gramicidin D. The H⁺-pumping activity in proteoliposomes was hardly affected by the elicitor. The finding suggests that a putative target molecule of or receptor for the elicitor might not associated with the solubilized ATPase or that the elicitor could not reach the target site. However, the suppressor markedly inhibited both activities in proteoliposomes, showing that the H⁺-translocating ATPase might be inhibited directly in vitro by the suppressor as reported previously and that the fungal suppressor may disturb the regulation of pH in the host cells. Both activities were increased by the addition of phosphatidylinositolbisphosphate, even if in the presence of the suppressor. However, PIP₂ could not completely negate the effect of the suppressor. These results suggest that the H⁺-pumping activity of plasma membrane ATPase is also regulated by polyphosphoinositide metabolism and that the action sites of the suppressor on the ATPase may be different from those of PIP₂.