The transport behavior of basic amino acids (BAA), such as arginine (Arg), histidine (His), and ornithine (Orn), through an organic liquid membrane system (LMS) was investigated. The LMS was composed of two aqueous phases (Phases I and II) separated by an organic phase of chloroform containing sodium di-(2-ethylhexyl) sulfosuccinate (Aerosol OT, AOT). The amount of BAA that moved from Phase I at pH 3 into the organic phase increased with increasing AOT concentration (2-10 mM). The relative amount of extracted BAA was in the following order : Arg > His > Orn. On the other hand, the release of BAA from the organic phase into Phase II at pH 10 did not depend upon their amount in the organic phase. Arg was difficult to release. The relative amount of released BAA was in the following order : Arg = His > Orn. BAA were extracted from Phase I at pH 5 into the organic phase containing 4 mM AOT because they exist as cationic species. Other amino acids possessing nonionic residues were untransportable under these conditions except leucine, tryptophan, and phenylalanine, which have highly hydrophobic residues. However, they were transportable in their cationic forms at pH 1. These transport phenomena are essentially controlled by the interaction of the anionic group of AOT and a cationic form. These results suggested that BAA can be separated from most amino acids under an appropriate pH by using AOT.