Direction: β-Galactosidase bound to DEAE-cellulose is used to hydrolyze lactose to glucose and galac lose in a plug fiow bioreactor with a packed bed of volume 100 liters and a voidage (ε) of 0.55. The K′_m and V′_max for the immobilized enzyme are 0.72 gl^–1 and 18 gl^–1 h^–1, respectively. The lactose concentration in the field stream is 20 gl^–1, and a fractional conversion of 0.90 is desired. Diffusional limitations may be ignored.