Recently, plant hairy roots have become of interest because of their active proliferation in a phytohormonefree medium and their capacity to produce valuable metabolites accumulated at comparable levels of the original plant roots. The purpose of our work is to extend the biotechnological application of hairy root cultures through studies on the following subjects. 1) Induction and characterization of hairy roots: Some hairy root clones were induced from red beet (Beta vulgaris), madder (Rubia tinctorum) and horseradish (Armoraeia lapathifolia) by Agrobacterium rhizogenes infection. These hairy roots had high contents of pigments or enzymes. 2) Bioreactor configuration: A turbine-blade fermentor was developed as a bioreactor suitable for the submerged culture of hiary root with a highly branched morphology. In this fermentor, a stainless steel mesh separated the culture space from the agitation space so that the hairy root cells were not in direct contact with the impellers. 3) Extracellular production of pigments: In the cultures of red beet and madder hairy roots, significant amounts of pigments were released from the cells into medium when the cells were subjected to the treatment of O₂ starvation. Both hairy roots were capable of growing even after the treatment. 4) Morphological change of hairy root: By cultivating the horseradish hairy root in the presence of phytohormones, the cell morphology could be changed reversibly between dedifferentiated-cell aggregates and organized hairy root.