In this study, comparative replicational properties of Hyphantria cunea nucleopolyhedrovirus (HycuNPV) in Lymantria dispar (IPLB-LdEta) and Spodoptera frugiperda (IPLB-Sf21) cell lines were investigated. Our microscopic observations showed that cytopathic effects (CPEs) in LdElta cells appeared 12 h later than those in Sf21 cells. Whereas polyhedral inclusion bodies (PIBs) formed at 48 h postinfection (p.i.) in LdElta cells, it formed at 36 h p.i. in Sf21 cells. Extracellular virus production determined according to the 50% tissue culture infective dose (TCID50) method in LdElta cells started about 12 h later when compared with Sf21 cells. Titers of extracellular virus in LdElta and Sf21 cells were calculated as 1.77 x 10(9) plaque forming units (PFU)/ml and 5.6 x 10(9) PFU/ml, respectively, at 72 h p.i. We also showed that viral DNA replication began at 12 h p.i. in both cell lines. Viral protein synthesis was determined by SDS-polyacrylamide gel electrophoresis (PAGE) and polyhedrin synthesis was observed at 12 h p.i. in both cell lines. The results indicate that while the synthesis of macromolecules is 12 h later and production of extracellular virus is almost 3-fold lower in LdElta cells compared with those in Sf21 cells, the LdElta cell line is still a productive cell line for infection of HycuNPV.