The antigenicity and immunogenicity of the intracellular and extracellular forms of vaccinia virus. I: The production of high-titre vaccinia extracellular virus and its antigenicity after inactivation.

Vaccinia extracellular virus, which until recently (Payne, 1979) had only been produced at low titre, 10(5)-10(6) pfu/ml, was produced at titres of up to 10(7.5) pfu/ml by selective combination of virus, cell and medium. It could be concentrated to 10(9)-10(9.3) pfu/ml by ultrafiltration without significant losses in quantity or quality. The final preparation was only contaminated to the extent of 25-50% with virus neutralizable by antiserum to intracellular virus, compared with the 70-90% found by other workers. When inactivated it produced good antibody titres in rabbits. In contrast, mice failed to produce antibody to extracellular virus, but did so to intracellular virus: they were also protected against a potentially lethal challenge with ectromelia virus.