The effect of NAA (1-naphthaleneacetic acid) treatment on emergence of adventitious roots was examined in the process of root regeneration using cell aggregates derived from horseradish hairy roots. The number of emerging roots was significantly larger from those cell aggregates with increasing amounts of NAA absorbed by the cells. The cell aggregates with absorbed NAA of 11.3 to 17.1 x 10(-3) kg/kg-dry cells formed 3.4 to 3.7 x 10(5) roots per M2-aggregate surface area during the culture period of 456 h, indicating an 8- to 9-fold increase over that of cell aggregates without the treatment. The large number of available roots emerging from cell aggregates with the NAA treatment consequently led to enhanced rates of root growth and sugar consumption in a batch culture for root regeneration. The kinetic behavior of root regeneration culture was described successfully on the basis of a model of branching root growth.