Vaccinia virus is a large and complex virus that produces two types of infectious virus particles, termed intracellular mature virus (IMV) and extracellular enveloped virus (EEV). EEV contains an extra lipid envelope and ten associated proteins that are absent from IMV. Although EEV represents less than 1% of infectious progeny it is very important biologically. First, it mediates virus dissemination and second, it is the virus against which protective immune responses are directed. This article reviews the genes known to encode EEV proteins and their functions, describes recent data showing that the cellular receptors for IMV and EEV are different, and demonstrates that EEV, in contrast to IMV, is resistant to neutralisation by antibody.