An acyclic diterpene alcohol, crinitol [1], was identified in a marine brown alga, Sargassum tortile, as the principal antibacterial agent against Gram-positive bacteria, among which Propionibacterium acnes was most sensitive and, Staphylococcus aureus was least. To enhance its activity, crinitol was combined with several antioxidants, which presumably retard oxidative destruction of this molecule which possesses two easily oxidizable allylic alcohol groups. Two synthetic antioxidants, BHA (butylated hydroxyanisole) and BHT (butylated hydroxytoluene), significantly enhanced the activity of crinitol, especially against Streptococcus mutans. Interestingly, crinitol also synergized BHT and BHA against this cariogenic bacterium.