All micro-organisms possess a barrier which effectively preserves the integrity of the organism and limits the entrance and exit of solutes. It is now generally agreed that this barrier, which has been termed the ‘osmotic barrier’, resides in the lipid-protein cytoplasmic membrane. Several pieces of evidence have led to this conclusion, for example, lipid-soluble compounds are often able to penetrate micro-organisms more quickly than lipophobic compounds. Moreover, the osmotic barrier can be broken by treating micro-organisms with lipid solvents, such as aqueous butanol, which cause the release of low molecular weight compounds from organisms. Microscopical observations on plasmolysable micro-organisms also indicate that the osmotic barrier resides in the cytoplasmic membrane.