The outermost layers of animal viruses are usually composed of glycoproteins. They are responsible not only for the entrance of viruses into, and release from host cells but also for the initial interaction of a viral particle with immunological defense of the host. It is therefore not surprising that many laboratories devote a lot of effort to study viral glycoproteins at the molecular level. Very often such studies are possible only after the introduction of a glycoprotein gene into a heterologous system. Expression of glycoprotein genes is usually obtained in mammalian or insect cells. Expression in mammalian cells yields viral glycoproteins with glycan chains indistinguishable from the original counterparts in virion particles but the level of synthesis of glycoproteins is very low. Vaccinia virus is the most common vector for expression in mammalian cells. It is easy to grow, the introduction of foreign genes is relatively simple and, due to the size of the vaccinia genome, it can accept large pieces of foreign DNA. Glycosylation in insect cells is not as complex as in mammalian cells and usually glycoproteins produced in insect cells are of slightly lower molecular mass than those produced in mammalian cells. The most common vector for expression of glycoproteins in insect cells is a baculovirus, Autographa californica nuclear polyhedrosis virus (AcNPV). The great advantage of this system is a very high level of expression of foreign genes.