The assembly of virion surface glycoproteins by enveloped viruses appears to be both specific and non-specific. It is non-specific in the sense that, during a dual infection by various unrelated viruses, there is very common (if not universal) and efficient mixing of envelope glycoproteins from both genotypes in the progeny virions. However, it seems to be specific in the sense that non-viral, cell surface proteins are, in the main, recognized as alien and are excluded from incorporation into virions during budding. These findings suggest that all enveloped viruses may share an essentially common molecular mechanism for the specific assembly and budding of virus glycoproteins while at the same time incorporating a mechanism for the exclusion of glycoproteins of cellular origin. One of the simplest explanations for this phenomenon may be that virus envelope surface structures all evolved from a single ancestral virus.