Immunoelectron microscopy was employed to examine the temporal expression and localization of two proteins involved in baculovirus polyhedron assembly (polyhedrin and p10) of Orgyia pseudotsugata multicapsid nuclear polyhedrosis virus (OpMNPV) in infected Lymantria dispar cells. In addition, the association of p10 with the polyhedron envelope (PE) protein was studied. The major capsid protein (p39) was also examined to investigate the association of virion structural proteins with polyhedron formation. In infected cells, p39 did not show a concentrated association with any infected-cell structures other than nucleocapsids and appeared to be randomly distributed over the nucleocapsid surface. Likewise, polyhedrin showed no major concentrations outside of developing or mature polyhedra. The p10 antibody cross-reacted with a protein associated with condensed chromosomes in uninfected cells. In infected cells, p10 is a component of the body of fibrillar structures. The PE protein has been shown to accumulate around the periphery of fibrillar structures. Cells infected with a polyhedrin-minus virus expressing the beta-galactosidase gene under the control of the polyhedrin promoter were examined to determine whether the lack of polyhedra would influence the localization of major polyhedron-associated viral proteins. High concentrations of PE protein accumulating on the periphery of fibrillar structures appeared to be the major difference from wild-type virus-infected cells. The beta-galactosidase protein appeared to be distributed throughout the nucleus and cytoplasm, in contrast with the specific localization of the viral proteins.