Two interferon components having different molecular weights were determined by gel filtration studies in non-purified rabbit serum interferon obtained 4 hours after intravenous administration of 200 μg poly I∶C. The high molecular component (M.W. ≧100,000) corresponded to a minor part (16.85 per cent) of the total interferon activity. The low molecular components has a molecular weight of 50,000–55,000 and was more heat stable than the high molecular component. By comparison with the interferon induced by Newcastle disease virus under the same conditions it appeared that, though the elution patterns from Sephadex G-100 of both the interferons were similar, there were differences in physicochemical properties of separate components: the high molecular poly I∶C-induced component was not affected by treatment with 4m urea (the high molecular component induced by NDV was completely inactivated) and it was also more heat stable than the corresponding component of the NDV-induced interferon.