The requirements for high level expression of three foreign proteins using the polyhedrin gene promoter of Autographa californica nuclear polyhedrosis virus (AcNPV, Baculoviridae) have been investigated. In Spodoptera frugiperda cells infected with the appropriate recombinant baculoviruses, the synthesis of the two S RNA coded genes of lymphocytic choriomeningitis virus (LCMV; i.e. the nucleoprotein, N, and glycoprotein precursor, GPC), or the haemagglutinin gene of influenza A virus, appears to be related to the degree of integrity of the 5' upstream sequence of the polyhedrin gene. No effect on the level of N protein expression was detected when all the polyhedrin gene coding sequences or some of the immediate 3' downstream sequences were deleted. Using the most efficient expression viruses derived from a new transfer vector, pAcYM1, it has been estimated that LCMV N protein represented approximately 50% of the total cellular protein, an observation consistent with the presence of numerous inclusion bodies in the cytoplasm of infected cells. For recombinant viruses derived from the pAcYM1 transfer vector containing the LCMV GPC gene, the level of synthesis of the arenavirus glycoprotein was equivalent to approximately 20% of the cellular protein. Thin sections of cells infected with the GPC recombinant revealed a highly vacuolated cytoplasm.