Abstract
Intact particles of Friend leukemia virus derived from infectious mouse serum absorb only trace amounts of cytotoxic anti-FMR antibodies, but physical disruption of the virions by freezing and thawing, by ether extraction or by detergent treatment releases large amounts of FMR antigenic activity. Thus this antigen, previously considered to occur mainly as a neo-antigen on the surfaces of virus-infected cells and as a soluble substance in the serum of infected mice, may be primarily a virion component.