Recently, it has been demonstrated that hairy roots induced by Agrobacterium rhizogenes were able to produce secondary products characteristic of the roots of original plant and cultures were established by the inoculation of Agrobacterium rhizogenes (A4, 15834) on such sterile medicinal plants as Abrus precatorius, Cassia torosa, C. obtusifolia, C. occidentalis, Nicotiana tabacum and Atropa belladona. All hairy roots cultivated so far showed positive opine production. Alkaloid contents of hairy roots of N. tabacum and A. belladona, were nearly the same or higher than those of normal root of original plants. Regenerated plants of N. tabacum showed wide variety of phenotypic alterations depending on the strains of isolated hairy roots. The phenotype showing the most wrinked leaves contained the same high amount of nicotine (1% dry wt.) as those in the decapitated plants. Cassia spp. was used as mild purgative drugs in oriental medicine. Original plant of Cassia torosa produced numerous anthraquinoids, naphtoquinones and xanthones. Cell culture of C. torosa produced germichrysone, a tetrahydroanthracene derivative, in high yields of 0.6∼1 ㎎/g fr wt, where as the hairy root culture produce pinceline, a xanthone derived from anthraquinone, in yield of 1∼1.2 ㎎/g fr wt. Cassia obtusifolia hairy root produced 1,8-dimethylchrysophanol as well as emodin, chrysophanol and parietin. Modified leaf disc method made feasible to establish hairy root culture even when an axenic plantlet was not available as in the case of Panax ginseng. The contents of ginsenosides (Rg₁, Rf, Rd, Rc, Rb₁, and Rb₂) were determined by HPLC in hairy root cultures of P. ginseng. Kinetic studies of growth and saponin production will be discussed. Direct infection and modified leaf disc method made feasible to establish hairy root culture even when an sterial plantlet was not available as in the case of Glycyrrhiza uralensis. The contents of Glycyrrhizin were determined by HPLC in hairy root culture of Glycyrrhiza uralensis. Kinetic studies of growth and saponin production will be discussed. The results of incorporation experiments using stable isotope revealed the potential of transformed hairy roots to be served as a system for biosynthetic study.