Different forms of acetylcholinesterase (AChE), EC 3.1.1.7, were demonstrated in human brain caudate nucleus. One form was solubilized at high ionic strength, the other with Triton X-100. The detergent-extractable form was purified to homogeneity by affinity chromatography. This form of AChE is amphiphile-dependent; i.e., it was active only in the presence of amphiphiles (detergents or lipids). Further, the enzyme was shown to bind detergents and to interact hydrophobically with Phenyl-Sepharose. In the presence of detergents the enzyme is a tetramer (subunit molecular weight, 78,000) which aggregates on the removal of detergents. Human brain AChE showed a reaction of identity with human erythrocyte AChE in crossed-line immunoelectrophoresis. The high-salt-soluble brain enzyme did not cross-react with the erythrocyte enzyme. The two classes of AChE seem not to be related, as they show no common antigenic determinant.