Silk fabrics were dyed in an extract of fresh leaves of the Japanese indigo plant, Polygonum tinctorium Lour., under various conditions. After dyeing, the dyes on the fabrics were extracted with dimethylsulfoxide. HPLC analysis of the extract indicated the presence of indigo (blue dye) and indirubin (crimson dye) in the dyed fabrics. The ratios of the amount of indirubin ( M _ir) to that ofindigo ( M _in) in the dyed fabrics were determined for the dyed specimens. The relationship between M _ir/ M _in and dyeing conditions was discussed. The favourable conditions for dyeing silk fabrics a red-purple shade are as follows : 1) the presence of 20-30% ethanol in the dyeing solution, 2) the pretreatment of the silk fabrics with alkaline such as sodium carbonate aq. solution, 3) a relatively high dyeing temperature.