The study was made of the antibacterial effects of three terpene alcohols on Staphylococcus aureus, focusing on the leakage of K+ ions and toxicity over time. The leakage of K+ ions was monitored continuously with a K+-electrode. Our results suggested that the terpene alcohols, namely, farnesol, nerolidol and plaunotol might act on cell membranes. The rank order of effectiveness, farnesol>nerolidol>plaunotol, was the same in the toxicity assay and in the examination of the leakage of K+ ions, when we considered the initial rate and the amount of leaked K+ ions. The rank order agreed with the results of a growth-inhibition assay reported previously. The antibacterial activity reflected the initial rate of leakage of K+ ions, suggesting that damage to cell membranes might be one of the major modes of action of these terpene alcohols. The results also demonstrated that the initial rate of leakage and the amount of leaked K+ ions are useful as indices of the antibacterial activities of hydrophobic compounds.