For the purpose of establishing a method of fixing the dyes used in supravital staining of cells, the author observed the precipitation reactions of several dyes, neutral red, Janus green B, Nile blue, etc. with the Nessler's reagent, by which those dyes having amino-, monomethylamino or dimethylaminogroups can be precipitated forming the water insoluble complexes. The reaction proved to have no relation with the oxidation or reduction of dyes, which often causes the change in the color of dyas, showing that the insoluble precipitates have the original color of dyes, and the colored dye forms the same colored precipitate, the reduced noncolored dye a white precipitate. The organic solvents such as alcohol, aceton etc. dissolve this complexes, and therefore, it is suggested that their reaction can be applicable for the fixation of the supravitally stained dyes with exception of the organic solvents in the course of the following dehydrating processes for the observation.