L-[5-3H]Tryptophan was administered to young plants of Isatis tinctoria and Polygonum tinctorium. Labelled indoxyl derivatives could be isolated from stems and roots of both plant species. Stems of I. tinctoria are able to convert [3H]-labelled tryptophan into indican and isatan B. Root cultures of I. tinctoria synthesize indican and isatan B and incorporate biosynthetic precursors into indoxyl derivatives. [2-14C]Indole but not D,L-[2-14C]tryptophan was incorporated by five-day-old etiolated seedlings of Isatis tinctoria into isatan B [5]. We report here in further experiments to elucidate the biosynthetic origin of indoxyl derivatives in woad and Polygonum tinctorium.