Allicin (diallyl thiosulfinate) is the main biologically active component of the freshly crushed garlic extracts. In the present work the ability of allicin to cross through membranes (artificial and biological) was studied. Partition coefficients of allicin in water/octanol, water/hexadecane and water/phospholipids mixtures were determined. Using phospholipid vesicles loaded with hydrophilic thiols (reduced glutathione or 2-nitro-5-thiobenzoate), we observed that allicin freely permeates through phospholipid bilayers and interacts with the SH groups. The reaction rate of allicin with SH containing molecules after crossing the membrane was the same as in solution. Fast diffusion and permeation of allicin across human red blood cell membranes was also demonstrated. Allicin does not induce leakage, fusion or aggregation of membrane. The high permeability of allicin through membranes may greatly enhance the intracellular interaction with thiols.