Protein yields in the baculovirus expression system do not always correlate with the presence of abundant amounts of corresponding mRNAs. Therefore, a novel aspect of the baculovirus expression system was studied : initiation of translation of very late mRNAs of Autographa californica multicapsid nucleopolyhedrovirus. The untranslated regions (UTRs) of the p10 mRNA of this baculovirus were studied by in vitro translation and after transfection into Spodoptera frugiperda insect cells. Lysates from insect cells were optimized for translation of in vitro transcripts containing p10 sequences. The lysates were used to measure the effects of various deletions in either the 5h or 3hUTR on protein synthesis. Transcripts containing the p10 5hUTR were translated efficiently. Large deletions in the 5hUTR severely decreased this efficiency. Deletions