Abstract: A luminescent substrate preparation having a concentration of catalytic inhibitors of less than about 100 ppm. The preparation is obtained by heating commercial grade luminol in a basic solution, crystallizing the luminol and separating the luminol crystals from the boiled solution. The heating, crystallization and separation steps are preferably repeated sequentially at least four times, with the starting material for each sequence after the first being the luminol preparation produced in the previous sequence. The luminol preparation has an enhanced pattern of activity, in that light output is substantially constant over a period of at least about one hour, with the intensity of light emitted by the preparation being at least about ten times that of commercially available luminol. Because of these enhanced characteristics, the luminol preparation is particularly adapted for use as a tag in specific binding assays where the concentration of analyte to be detected is low.