The roots of <i>Mercurialis leiocarpa</i> (Sieb. et Zucc.) contain a blue dyestuff found to be cyanohermidin resembling allagochrome, a copper protein, by one of the present authors. This paper describes the isolation and chemical nature of the dye. Electrophoresis for purification, dialysis for extracting copper, and measurement of its molecular weight by gel permeation showed that it readily changed into smaller molecules due to the loss of copper and that it was considered to be a complex of anionic radicals of 4-methoxy-1-methylpyridine-2, 3,6-trione and its tautomer, 3, 6-dihydroxy-4-methoxy-1-methyl-2-pyridinone containing copper atoms as a stabilizer. The ancient dyeing techniques using this dyestuff were also discussed. Continued from earlier study (see <i>AATA</i><b> 20-2270</b>). -- AATA