Recombinant L-929 cells transfected with the human erythropoietin (EPO) gene were immobilized in a macroporous cellulosic support and its derivatives in which charged groups or cell attachment factors were introduced. The immobilized cells were cultured in serum-containing and serum-free media. Comparable production of EPO was observed even in the serum-free medium when a support modified by polyethyleneimine was used for immobilization. The cells immobilized on the supports were cultured in fluidized-bed and inner-loop type air-lift bioreactors for continuous production of EPO. A high cell density of more than 2 x 10(7) cells/cm3-support and high EPO productivity were achieved and maintained for 50 d through the use of the inner-loop type air-lift bioreactor. The productivity was 13.4-fold higher than that of conventional static cultures in petri-dishes.