To characterize the infection cycle of the Orgyia pseudotsugata multicapsid nuclear polyhedrosis virus in Lymantria dispar cells, the time course of DNA synthesis and polyhedron production, and the onset and rate of budded virus production were investigated at three different m.o.i. (5, 10 and 100). In addition, the time course of expression of three proteins (gp64, p39 and polyhedrin) representative of three temporal classes of baculovirus genes was also analysed using Western blot analysis. DNA synthesis began at 12 to 18 h post-infection (p.i.). The rate of budded virus (BV) production reached maximal levels at 24 to 36 h p.i. and continued at high levels indicating that BV production was not turned off late in infection. Polyhedra were first observed at 48 h p.i. The m.o.i. appeared to influence the magnitude but not timing of early events in the viral infection cycle (gp64 expression and DNA synthesis) and also influenced the initial levels of BV production and the percentage of cells containing occlusion bodies. The m.o.i. had little influence on the final rates of BV production and the time of detection of p39 and polyhedrin on Western blots.