Indirubin, a red isomer of indigo, is sometimes formed when dyeing with fresh leaves of indigo plants. Herein, conditions for the production of indirubin were investigated. Indirubin was rapidly formed from the reaction of indoxyl acetate with isatin under alkaline conditions. Even without the addition of isatin, indoxyl acetate yields much indirubin under alkaline conditions at higher temperatures. Production of indirubin from indoxyl acetate in silk fabrics could be carried out by pre-treating silk with alkaline solution. Moreover, silk fabrics could be dyed a purplish color from a hydrolyzed solution of indican under alkaline conditions.