The time course of virus replication in cultures of permissive cells infected with high multiplicities of herpesvirus saimiri (HVS), a gammaherpesvirus, is protracted relative to the replication of herpes simplex virus (HSV), an alphaherpesvirus, under similar conditions. The basis for this difference was investigated by quantitative immunofluorescence microscopy exploiting monoclonal antibodies specific to the HVS 52 000 mol. wt. immediate-early polypeptide (IE 52K) and to delayed-early (DE 51K, DE 110K) and late (130K and capsid proteins) gene products to measure the timing of gene expression in individual cells of infected cultures. The timing of the transitions from IE to DE and from DE to late protein synthesis occurred at proportionately different intervals in the growth cycle of HVS, relative to that of HSV. In particular, the DE to late transition occurred relatively later in HVS infections. However, asynchrony in the events leading to the expression of the first class of virus proteins (IE 52K) was the main source of the extended course of HVS replication in populations of infected cells. This asynchrony was not modified significantly by infection at different stages of the host cell-cycle and was reduced, but not overcome, by very high applied multiplicities of infection. Double-antibody staining revealed a positive correlation between the accumulation of high concentrations of parental virus particles at perinuclear sites and early detection of HVS IE 52K gene expression. Both of these events remained sensitive to a microtubule poison (colcemid) for many hours after infection with HVS, whereas the rapid and synchronous expression of the IE 175K protein (ICP4) in HSV-1-infected cells was insensitive to post-infection exposures to this drug. We conclude that significant differences in early stages of virus entry and intracellular processing which precede immediate-early gene expression are largely responsible for differences between the replicative cycles of these representatives of gamma- and alphaherpesviruses in cultures of permissive cells.