Mouse serum interferon (MSIF) induced by intravenous injection with Newcastle disease virus (NDV) was subjected to purification by using Sephadex G-100 gel filtration and DEAE cellulose column chromatography. Purified IF fractions were characterized by polyacrylamide gel disc electrophoresis (PGDE) and immunodiffusion technique. Either on Sephadex G-100 gel filtration eluting with 0.1M phosphate buffer (pH 7.4) containing 0.001 M EDTA, or DEAE cellulose column chromatography eluting with 0.05 M Tris-HCl buffer (pH 8.4) containing 0.001 M EDTA and gradual increasing molarity of NaCl, two peaks were found to be associated with IF activity. The first peak from Sephadex gel filtration (I100) showed an ambiguous band located between alpha2 and beta-globulin on PGDE, while the second peak on DEAE chromatography (IID) showed no particular band on PGDE. Both the second peak which appeared on Sephadex gel filtration (II100) and the first peak appeared on DEAE chromatography (ID) presented the clear band between alpha1 and alpha2-globulin. Among these four peak fractions, ID was least contaminated by other serum protein components. Immunodiffusion analysis of whole MSIF against anti-MSIF revealed that there was only one particular band which was probably identical to II100 and ID that appeared in Ouchterlony's plate. Whole MSIF was also analysed by using cellulose acetate membrane electrophoresis (CAME). The pattern of CAME showed that the percentages of area at alpha1 and beta-globulin on the recording trace for MSIF seemed to be larger than those of control normal mouse serum.