Ultrastructural localization of acetylcholinesterase activity by means of the electron dense precipitate derived from Koelle's cuprous thiocholine iodide by treatment with phosphomolybdic acid and osmium tetroxide.

An osmium resistant, thermostable and electron dense precipitate was obtained from cuprous thiocholine iodide (Koelle's precipitate) by a joint interaction with phosphomolybdic acid and OsO4. No diffusion artifact due to the conversion of the primary precipitate to the secondary precipitate was observed, contrary to that seen after (NH4)2S or K3Fe (CN)6 posttreatment of the cuprous thiocholine iodide. In addition, phosphomolybdic acid and OsO4 provided a counterstain effect on the ultrastructural background. By the present modification, Koelle's histochemical method becomes a useful cytochemical method for ultrastructural localization of acetylcholinesterase activity.