Recombinant human Factor IX (rFIX) was cloned in a mammalian expression vector and transfected into CHO and HEK-293. Treatment with 10(-9) M methyl testosterone increased rFIX production by 30-50% in CHO and HEK clones. However, 10(-9) M 17 beta-oestradiol increased production of rFIX by similar to 50% in CHO-F7 clone and decreased production by 48% and 37% in CHO-F8 and HEK-F2-6, respectively. Progesterone treatment inhibited rFIX production in both cell lines. Production of rFIX can thus be increased by sex hormone treatment and therefore used to enhance biotechnological production in mammalian cells.