Baculovirus vectors have been shown to exhibit not only a highlevel of gene expression in insect cells but also efficient gene transduction into a wide variety of mammalian cells with lower cytotoxicity (Hofmann et al., 1995; Boyce and Bucher, 1996; Shoji et al., 1997) . In contrast, the complement systems of animals have been defined to represent a potent primary barrier to in vivo application of baculovirus vectors produced in insect cells (Hofmann and Strauss, 1998; Tani et al., 2001 Tani et al., , 2003 . However, pseudotype viruses based on retroviruses or lentiviruses bearing baculovirus envelope protein GP64 have recently been shown to exhibit efficient gene transduction into mouse organs for long periods compared with those bearing the VSV G protein, which is commonly used for pseudotyping (Kumar et al., 2003; Schauber et al., 2004; Kang et al., 2005; Sinn et al., 2005 Sinn et al., , 2008 . It was considered that serum resistance of the pseudotype viruses bearing GP64 protein is caused by differences between insect and mammalian cells, because pseudotype retroviruses or lentiviruses were generated from mammalian cells, and in contrast, baculovirus was generated from insect cells. Therefore, we generated recombinant VSV bearing GP64 protein in both insect and mammalian cells and examined the role of the GP64 on resistance to inactivation by human or guinea-pig sera . Recombinant VSVs generated in human cell lines exhibited the incorporation of human decay accelerating factor (DAF) in virions and were resistant to serum inactivation, whereas those generated in insect cell lines exhibited no incorporation of human DAF and were sensitive to complement inactivation. Recombinant baculoviruses generated in insect cells expressing human DAF or carrying the human DAF gene exhibited resistance to complement inactivation, suggesting that acquisition of resistance to human complement by the incorporation of DAF with baculovirus GP64 represents a step in the development of novel viral vectors for improved gene therapy. in the cells treated with various entry inhibitors depended on the species of pseudotype virus, suggesting that several entry mechanisms were involved in the infection of arenaviruses (Tani et al., unpublished data) . In studies on serological diagnosis of arenaviruses, cross reaction occurred among species of arenaviruses in enzyme-linked immunosorbent assay or immunofluorescence assay, whereas neutralization tests using pseudotype viruses exhibited a specific reaction with each species of virus (Nakauchi et al., 2009; Iha et al., unpublished data) . Although Old or New World arenaviruses have been shown to utilize α-dystroglycan or human transferrin receptor 1, respectively, as one of the cellular receptors, infectivities of the pseudotype viruses have not been consistent with the expression levels of the receptor molecules in our preliminary studies. The infection of pseudotype viruses was not completely inhibited by soluble protein or antibodies of receptor molecules, suggesting that another receptor molecule(s) might be involved in the entry of these viruses. Although further characterization of the pseudotype viruses bearing GPC envelope proteins of arenaviruses will be needed, these viruses are thought to mimic the functional properties of wild type arenaviruses and are suitable for the study of entry mechanisms, including investigation of novel cellular receptor(s), neutralization tests, or vaccine development.