Mouse plasma acetylcholinesterase (AChE) tetramers (G4) and dimers (G2) were retained by edrophonium-Sepharose, whereas AChE monomers (G1), and G4, G2 and G1 butyrylcholinesterase (BuChE) forms were not. Plasma G4 or G1 AChE did not differ in their affinity for edrophonium. G1 AChE, and G1 and G2 BuChE were retained in octyl-Sepharose, while G4 and G2 AChE, and G4 BuChE eluted freely. The amphiphilic behaviour of G1 AChE remained unmodified after incubation with trypsin. The electrophoretic mobility of the AChE monomers varied with the detergent added to the samples. The results show that mouse plasma G1 AChE possesses hydrophobic regions, which prevent its binding to the affinity matrix, and afford its interaction with octyl-Sepharose. The hydrophobic regions in G1 AChE probably provide conformational stability to disulfide-linked subunits in hydrophilic dimers.