Introduction. Recombinant DNA technology has made possible the expression of heterologous genes in a variety of animal viruses (for review, see Rigby, 1983). The availability of a wide variety of animal virus vectors enables a foreign gene to be expressed in different cell types, at different levels and with different consequences for the host cell. Factors influencing the choice of virus vector include the types of cells or animals to be infected, whether infection leading to cell transformation or lysis is required, the number of genes to be expressed, and whether virus infectivity is to be retained. Small DNA viruses (e.g. papovaviruses) may have restricted host range and limited capacity for foreign DNA due to severe packaging constraints imposed by the icosahedral virus capsid. Consequently these virus vectors are mostly replication-defective, requiring helper virus or special cell lines for their replication. In contrast, larger viruses (poxviruses and herpesviruses) have a greater capacity for foreign DNA without destruction of infectivity and can have a wide host range.