Human cytomegalovirus inhibits the growth of human foreskin fibroblast cells by 12 h after infection. Analysis of the cellular DNA content of infected cells by flow cytometry demonstrated that cytomegalovirus does not arrest cell cycle progression at a single point. At least two blockages occur, one of which is in the G(1) phase of the cell cycle. The G(1) arrest introduced by cytomegalovirus infection blocks S-phase entry after serum stimulation.