A novel baculovirus-based protein expression strategy was developed to produce recombinant proteins in insect cells without contaminating baculovirus virions. This novel strategy greatly simplifies the downstream processing of biopharmaceuticals produced in insect cells. The formation of these virions is prevented by deletion of a baculovirus gene essential for virion formation. The deletion is trans-complemented in a transgenic insect cell line in which the baculovirus seed stock is produced. The Autographa californica multicapsid nucleopolyhedrovirus vp80 gene was selected for this purpose, as absence of VP80 prevented the formation of budded virus as well as occlusion-derived virus, while foreign gene expression was not affected. Sf9 insect cells were engineered to functionally complement the vp80 deletion in the expression vector virus during seed stock production. The trans-complemented vp80-deletion baculovirus seed produced an amount of recombinant protein similar to that produced with conventional baculovirus vectors but without contaminating virions. This novel expression method obviates the need to purify the virions away from the biopharmaceuticals. Biotechnol. Bioeng. 2011;108: 1056-1067. (C) 2010 Wiley Periodicals, Inc.