VACV, the prototypical orthopoxvirus, produces two types of infectious virions that arebiologically and antigenically different. The majority of the virions produced are intracellular mature virions (MVs). MVs contain an envelope with more than 20 envelopeproteins. A fraction of MVs gain additional membranes inside the cells and eventually exitthe cells as the extracellular enveloped viruses (EVs). EV contains anadditional envelope with at least six envelope proteins. Antibodies against both MV and EV are required for optimal immune protection against orthopoxvirus. Among the EV proteins, B5 is the major target of neutralization antibodies, while A33 is the target of protective antibody. Depletion of ant-B5 antibodies from sera of vaccinated individuals greatly reduced in vitroneutralization of EVs. Among the MV envelop proteins, A27, L1, D8, H3, A28 and A17 are known to be the targets of neutralizing antibodies.