Abstract In the marine snail Murex brandaris about 80% of cholinesterase (ChE) activity lies in the blood. It can be recovered as a fully soluble (FS) form by mincing the whole animal. Two more ChE forms, detergent (DS) and high‐salt soluble (HSS) (18 and 2% of total ChE activity, respectively), can then be sequentially extracted from the other tissues. FS and DS ChEs were purified to homogeneity by affinity chromatography on procainamide‐or edrophonium‐Sepharose respectively. The small amount of HSS prevented a similar purification and an extensive characterization. According to density gradient centrifugation, gel‐filtration chromatography, and SDS‐PAGE, FS ChE is likely a globular tetramer of a 66 kDa subunit (10.8 sedimentation constant [S], 270 kDa). Moreover, it is an amphiphilic form including a hydrophobic domain. DS ChE appears to be a globular dimer of a 66 kDa subunit (5.6 S, 137 kDa). The amphiphilicity of this enzyme is likely due to a phosphatidylinositol on the catalytic subunits, also responsible for detergent interaction as well as cell membrane insertion. Both FS and DS forms hydrolyze propionylthiocholine faster than other choline thioester substrates. They also show high catalytic efficiency with other choline esters as substrates, likely due to a wide and relatively unspecialized conformation of the active site. Immunological cross‐reactivity showed wide structural affinity between FS and DS forms. Antibody‐enzyme bond gave partial inactivation. On the whole, the results suggest that both FS and DS forms likely originate from only one gene. Differences in quaternary structure and solubility could reflect posttranslational modifications or alternative splicing. © 1994 Wiley‐Liss, Inc.