Treatment with 2 mM CuSO4 was used to induce a Drosophila melamogaster metallothionein (Mtn) promotor that had been cloned into a recombinant baculovirus. Careful study revealed that the Mtn promoter functioned as an inducible, if somewhat 'leaky' promoter within the context of baculovirus- infected cells. In the process of generating a recombinant-baculovirus, it was discovered that post-transfection treatment with copper resulted in a 10- fold increase in the production of recombinant virus. This effect on virus production was specific to transfection, as treatment of infected cells with copper did not increase the production of virus. Treatment of infected cells with copper did, however, extend the period of expression of the polyhedrin and p10 proteins by at least 12 h. These findings have practical applications for the production of recombinant baculoviruses and the subsequent expression of foreign proteins using baculovirus expression vectors.