Transient gene expression is one possible approach to manipulate the signaling pathways that control the proliferation and differentiation of human embryonic stem (hES) cells. We tested in hES cells a range of baculoviral vectors with a human elongation factor-la promoter and various viral regulatory elements and observed the most dramatic augmenting effect on the transient expression when the promoter was used together with the human cytomegalovirus immediate-early gene enhancer and the woodchuck hepatitis virus post-transcriptional regulatory elements. This vector provided a 1.6-fold increase in the percentage of transduced cells (up to 72%) over a vector containing the elongation factor-1 alpha promoter alone. The effective baculoviral transduction of hES cells did not affect cell proliferation, expression of embryonic stem cell markers and teratoma formation. This new viral vector for temporary transgene expression might become a useful tool for developmental biology studies and biomedical applications of hES cells. (C) 2009, The Society for Biotechnology, Japan. All rights reserved.