Membrane bound, Triton X-100 solubilized bovine nucleus caudatus acetylcholinesterase sediments in the presence of Triton X-100 concentrations higher than the CMC [critical micelle concentration] as a 10.5 S detergent-enzyme complex. This complex represents neither the molecular enzyme arrangement present in the membrane nor the molecular form originally released from the membrane. The purified, cytoplasmatic acetylcholinesterase sediments as a 10.5 S form also. This form is clearly distinguished from the detergent-enzyme complex, for it is not capable of aggregating, whereas the 10.5 S detergent-enzyme complex aggregates on detergent removal to defined water-soluble oligomers with sedimentation coefficients of 16 S (700,000 .+-. 10,000), 20.6 S (960,000 .+-. 60,000) and 23.3 S (.apprx. 1,200,000). In contrast to acetylcholinesterase from erythrocytes this aggregation is not easily reversible by incubation with Triton X-100, reflecting differences in the hydrophobic part of the enzymes. Purified acetylcholinesterase solubilized without detergent under autolytic or tryptic conditions sediments mainly as a 4.5 S form. Such slow sedimenting forms detected in crude solubilisates of neuronal tissues may originate at least partially from autolytic solubilization.