A method has been described for the large scale production of potent mouse interferon preparations based on the cultivation and induction of cells in suspension culture. By the selective use of metabolic inhibitors and the choice of optimal culture conditions, large quantities of interferon titering 4 × 105 mouse reference units/ml (specific activity 106 reference units/mg of protein), were consistently obtained. It was found that the inducing virus could be recovered at the end of the adsorption period and reutilized at least seven times without loss of its interferon inducing capacity. Although this method has been described for the production of mouse interferon using C243-3 cells it is applicable to the production of human or other interferons from cells which multiply in suspension culture.