Shimizu and Tsuji established a method of preparing colloidal platinum nanoparticles, whose average size is 2 nm, by ethanol reduction of H₂PtCl₆ in the absence of protective agents for the particles. Platinum nanoparticles have negative surface potential and are stably suspended from an electric repulsion between them. The platinum nanoparticles reduced hydrogen peroxide and 2,2-diphenyl-1-picrylhydrazyl radical (DPPH) below 0.1 ppm. It is necessary to use higher concentration of platinum nanoparticles for the reduction of 2,6-dichlorophenol indophenol (DCIP) than that of hydrogen peroxide and 2,2-diphenyl-1-picrylhydrazyl radical, because reoxidation of DCIPH₂ (reduced) by oxygen was not negligible under our experimental conditions. These results indicate that electrons on platinum nanoparticles produced by the method of Shimizu and Tsuji can reduce hydrogen peroxide, DPPH radical or DCIP transferring electrons.