Small amounts of Hatchett's brown (cupric ferrocyanide, Cu2Fe(CN)6·7H2O), deposited at the sites of cholinesterase activity in tissues by the procedure of Karnovsky and Roots (1964), may be enhanced by bridging to osmium through thiocarbohydrazide (TCH). Alternatively, amplification of the deposits may be attained by utilizing the cupric ferrocyanide as a catalyst to effect the oxidative coupling of 3,3′-diaminobenzidine (DAB). The resulting intensely colored osmiophilic polymer is more visible and after osmication, is electron opaque. With these procedures, the light microscope preparations are more permanent than with the Gomori modification of the Koelle and Friedenwald procedure.