Extracellular rabbitpox virus released naturally from infected cells differed antigenically from intracellular virus released by artificial disruption of cells. Intracellular virus was neutralized by antiserum prepared against live rabbitpox virus and by antiserum against inactivated vaccinia virus. In contrast, extracellular virus was neutralized only by rabbitpox antiserum. The antibodies responsible for the neutralization of intracellular and extracellular virus could be absorbed separately from rabbitpox antiserum. Morphologically, extracellular virus differed from intracellular virus in possessing an outer envelope. This envelope was probably the site of the virus antigen characteristic of extracellular virus, and fluorescent antibody staining of infected cells suggested that it was derived from the modified host cell membrane. Antibody directed against extracellular virus was responsible for the ability of rabbitpox antiserum to control the spread of rabbitpox virus in tissue culture and probably for its ability to protect rabbits from rabbitpox infection. Extracellular virus should therefore be used as the test virus in titrations of neutralizing antibody if these are to assess the protective activity of an antiserum.


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