The assymmetric 18S and 14S forms of acetylcholinesterase (EC 3.1.1.7) from Electrophorus electricus purified by affinity chromatography on N-methylacridinium Sepharose 2B were subjected to trypsin or collagenase proteolysis and changes in the enzyme composition and structure were monitored by sucrose gradient sedimentation, gel chromatography, and sodium dodecyl sulphate - polyacrylamide gel electrophoresis. A distinction between autolytic and tryptic degradation products is described and the generation of two new forms of acetylcholinesterase from the 18S and 14S enzyme by collagenase proteolysis is reported. The species derived from the 18S form of acetylcholinesterase has a sedimentation coefficient of 21.1S and a Stokes radius of 12.9 nm while the 14S form gives rise to a 17.3S species with a Stokes radius of 11.1 nm. The proteolytically sensitive component ('tail') of the asymmetric forms of acetylcholinesterase is identified with a subunit of 45 000 daltons on sodium dodecyl sulphate - polyacrylamide electrophoresis gels.