—The total AChE of brain can be readily extracted into aqueous Triton X-100. By column chromatography of these extracts a preparation was obtained at least six times as active as the original material and in yields of 60-80 per cent of the original amounts. The molecular weight of this material was estimated to be over 200,000. When brain tissue was treated with venom or bacterial protease, a water-soluble AChE was obtained with an overall purification of 150-fold. The most active preparation of AChE hydrolysed 880 μ-moles of acetylthiocholine/hr/mg of protein. The molecular weight of this preparation was estimated to be about 100,000. The enzyme which was extracted by Triton X-100 is probably the precursor of the more water-soluble enzyme that can be prepared from brain tissue by treatment with venom or protease.