Antigen-specific nonimmunoglobulin factor that neutralizes xenotropic virus is associated with mouse serum lipoproteins.

Abstract
A soluble nonimmunoglobulin factor that specifically neutralizes mouse xenotropic C-type virus is found in normal mouse sera. It is stable from pH 2 to neutrality and resists ether extraction, freezing or brief heating to 100.degree. C. It can be separated from immunoglobulins by ultracentrifugation at a density of 1.21 g/cm3. Neutralizing activity is only found with the serum lipoproteins in the fraction with density less than 1.21 g/cm3.