A hybrid recombinant baculovirus-bacteriophage T7 expression system was developed for transient expression in insect cells of plasmids with foreign genes provided with a 17 promoter. The coding sequence for 17 RNA polymerase, with or without a nuclear localization signal, was inserted into the genome of Autographa californica nuclear polyhedrosis virus. Recombinant viruses stably expressed 17 RNA polymerase in insect cells. Upon transfection of infected insect cells with plasmids containing the genes for chloramphenicol acetyltransferase (CAT), the hepatitis B virus precore-, core- or e-antigens under control of the T7 promoter, transient expression of these genes was detected by ELISA. The results obtained indicate that this baculovirus/T7 system provides a simple and widely applicable tool for transient gene expression studies.