The effects of water content in agar gel used as a medium and oxygen level in the gas phase on the adventitious root regeneration of cell aggregates (CA) derived from horseradish hairy roots were investigated in cultures at 25 degrees C. The number of roots emerging from CA was highly dependent on water content of the agar gel and no root regeneration was observed at a gel water content of 66% during culture time of 20 days. CA root regeneration was suppressed when the CA were kept for 20 days in an atmosphere oxygen composition of 10%, but was restored upon transfer of the CA to normal atmosphere of 21% oxygen. On the basis of these findings, an artificial seed was proposed using the CA as a cell inclusion material encapsulated in alginate gels covered with coats. From the perspective of conserving the root regeneration potential of the CA by preventing the drying of alginate gel while keeping oxygen availability to the CA, different coating materials of ethylene vinyl acetate acrylic acid terpolymer, paraffin and polyorganosiloxane were tested. Paraffin was selected as a suitable coating material because of its efficient drying tolerance and adequate permeability to oxygen. A regeneration efficiency of 90% could be obtained from the CA, stored in alginate gel covered with a paraffin coating of 0.40 mm thickness at 25 degrees C for 60 days in air, when sucrose concentration in the gel was over 240 mol m(-3).