The improved method presented here for localizing monophenoloxidase activity of tyrosinase (E.C. 1.14.18.1.) after electrophoresis is based on the transfer of electrons from the monophenolic substrate, tyrosine methyl ester, to an artificial acceptor, phenazine methosulfate, and subsequent reduction of nitro blue tetrazo-lium into a violet formazan. This method is rapid, sensitive and versatile compared to the standard method. The electron transferred from mono-phenol can be accepted directly by nitro blue tetrazolium; although the background of the gel is clear, the sensitivity is decreased. The mono-phenol-PMS-NBT method is suitable for both plant and animal samples. This method can also be used for histochemical demonstration of monophenoloxidase activity.