The existence of an envelope on extracellular cowpox virus and its antigenic relationship to the vaccinia envelope.

Virus released from cowpox infected cells was demonstrated by electron-microscopy to be surrounded by an envelope not present on mature intracellular virus. Enveloped cowpox had an isopycnic density of 1.23 g/ml, was infectious and neutralized by antibodies specific for the envelope antigens of vaccinia virus but not neutralized by antibodies specific for intracellular naked vaccinia virus. Only 1 per cent of the total intracellular virus yield was released as enveloped virus. The major cowpox glycoprotein (76 K) and a 44 K glycoprotein did not comigrate with any vaccinia glycoproteins whereas cowpox glycoproteins at 42 K and 20-23 K did coelectrophores with vaccinia glycoproteins. All of the mentioned cowpox glycoproteins were precipitated by vaccinia antiserum.