According to their solubilization properties, two classes of acetylcholinesterases (AChE) can be detected in the adult rat brain: a "soluble" species (easily solubilized without detergent), and a membrane-bound species (solubilized only in the presence of detergent). The latter was found to be homogeneous by gel filtration (Stokes radius 8.05 +/- 0.35 nm) and sucrose gradient centrifugation (9.75 +/- 0.2 S) in the presence of Triton X-100. The "soluble" AChE gives three stable species in the presence of the same detergent with Stokes radii and sedimentation constants of 10.9 +/- 0.5 nm and 16 +/- 2S; 6.75 +/- 0.30 nm, and 10.7 +/- 0.4 S; 5.37 +/- 0.35 nm and 4.37 +/- 0.1 S. Co-chromatography and co-sedimentation or the reduction and alkylation of disulfide bridges show that all the soluble species are different from the membrane-bound AChE. The possibility that soluble and membrane-bound AChE are completely different molecules is discussed.