Two major late proteins, polyhedrin and p10, are synthesized in large quantities in baculovirus infected insect cells. This and the fact that both proteins are dispensable for virus replication, form the basis for the use of these viruses as vector for foreign gene expression. To address the question whether the Autographa californica nuclear polyhedrosis virus p10 promotor-driven expression is influenced by the concurrent expression of the polyhedrin gene, several recombinants were constructed with various deletions in the polyhedrin gene. The Escherichia coli lacZ gene was used as a marker to allow direct comparison between p10 and polyhedrin-driven expression. None of the deletions in the polyhedrin gene did result in higher expression of the p10 promoter-controlled gene. This suggested that the transcriptional and/or translational activity of the p10 and polyhedrin gene are independently regulated. To compare the level of polyhedrin and p10 promoter driven expression, recombinants with the lacZ gene cloned behind either promoter were studied. No significant difference in level of expression was observed. In cells infected with a recombinant with the lacZ gene present behind both promoters a reduced level of expression was observed, whereas a considerable increase was expected. This may be due to instability of the viral genome, as two copies of the lacZ gene were present.