The antagonistic effects of the fungal toxin beticolin-1 and of L-α-lysophosphatidylcholine (lysoPC) were investigated on the plasma membrane H⁺-ATPase of the plant Arabidopsis thaliana (isoform 2) expressed in yeast, using both wild-type enzyme (AHA2) and C-terminal truncated enzyme (aha2Δ92). Phosphohydrolytic activities of both enzymes were inhibited by beticolin-1, with very similar 50% inhibitory concentrations, indicating that the toxin action does not involve the C-terminal located autoinhibitory domain of the proton pump. Egg lysoPC, a compound that activates the H⁺-ATPase by a mechanism involving the C-terminal part of the protein, was found to be able to reverse the inhibition of AHA2 by beticolin-1. The lack of effect of other detergents and the comparison of different carbon chain length lysoPCs show that the capacity to reverse the enzyme inhibition is clearly related to their ability to activate the pump. Long chain length lysoPC was also shown to reverse the inhibition of aha2Δ92 by beticolin-1, which strongly suggests that lysoPC binds to the H⁺-ATPase on site(s) not located on its autoinhibitory domain.