Because several observations have suggested that replication of the gammaherpesvirus bovine herpesvirus 4 (BHV-4) is influenced by the physiological state of the host cell, a study was carried out to determine the relationship between BHV-4 infection and the cell cycle. The temporal expression of BHV-4 late (L) proteins in unsynchronized cell cultures was first investigated by flow cytometry. Interestingly, L protein expression occurred in a limited number of cells infected with a high multiplicity of infection, and a reciprocal correlation between the percentage of positive cells and the cell density at the time of infection was demonstrated. Moreover, the finding that a BHV-4 early-late protein was expressed in nearly all the cells suggested that a blockage in the viral replication cycle occurred in some infected cells at the stage of viral DNA synthesis or L protein expression. Because this blockage could be the consequence of the dependence of one or both of these events on the cell cycle, they were investigated after infection of synchronized cell cultures. The following findings were made. (i) Cell transition through the S phase quantitatively increased the rate of BHV-4 DNA replication. (ii) BHV-4 DNA synthesis could not be detected in cells arrested in G0. (iii) Synchronization of MDBK cells with Lovastatin before infection increased the percentage of cells expressing L proteins. (iv) In contrast, infection of cells arrested in G0 led to few positive cells. Taken together these results showed that BHV-4 DNA replication and consequently the expression of L proteins are dependent on the S phase of the cell cycle. This dependence could be of importance for several biological properties of BHV-4 infection in vitro and might have implications for the biology of the virus in vivo.