Cell culture. Callus of Oryzae sativa L. cv. BL-I was kindly supplied by Mr. Masahiro Kobayashi of Chugai Pharmaceutical Company and maintained on agar plates containing N6 medium with 1 mg/liter 2,4-dichlorophenoxyacetic acid (2,4-D) and 0.8% agar. The callus was incubated at 25°C in the dark and transferred to fresh medium every 4 weeks. For suspension culture, approximately 4 g of the callus was transferred to a 500-ml Erlenmeyer flask containing 150ml ofN6 medium containing 1 mg/liter 2,4-D. The suspension culture was incubated on a rotary shaker at 25°C and 140 rpm in the dark. Cells were harvested every 2 weeks and filtered through a 20 mesh filter to make fine aggregates. A lO-ml sample of the loosely packed cells was transferred to a new flask containing l50ml of the same medium and incubated further.