Hydrogen peroxide amplifies the chemiluminescence in the oxidation of luminol by sodium hypochlorite. A linear relationship between concentration of hydrogen peroxide and light intensity was found in the concentration range 5 x 10(-8)-7.5 x 10(-6) mol/l. At 7.5 x 10(-6) mol/l H2O2 the chemiluminescence is amplified 550-fold. The chemiluminescence spectra of these reactions have a wavelength maximum at 431 nm independent of the concentration of hydrogen peroxide. The results indicate that hydrogen peroxide is a necessary component in the chemiluminescent oxidation of the luminol by sodium hypochlorite.