The activity of HuIFN-alpha 2 produced in bacteria has been compared with that of HuIFN-alpha from Namalwa cells (HuIFN-alpha N) as an antiviral (against EMC virus) and antiproliferative agent in normal and malignant cultured human breast cells. The IFN preparations show the same spectrum of antiviral and antiproliferative activity in the human cell strains and lines examined, and are equally effective, when comparable amounts of IFN protein are used. Both interferon preparations inhibit virus growth (EMC) in four types of bovine cells but neither inhibit the growth of these cells. HuIFN-alpha 2 and HuIFN-alpha N can also be shown to effectively inhibit the growth of a transplantable human breast cancer, grown as xenografts in the nude mouse, although the amounts of IFN protein required may not be the same for both preparations.