In vitro incubation of human cytomegalovirus (Towne strain) with 8 U/ml human recombinant myeloperoxidase plus sodium chloride and glucose nearly abolished viral infectivity. To assay the effect on intracellular infection, cell toxicity of the enzymes was first studied. Even the high dose of 16 U/ml of recombinant myeloperoxidase plus 10 mU/ml glucose oxidase did not decrease MRC5 cell growth. By contrast, this dose reduced proliferation of activated THP1 cells. Even half of the myeloperoxidase dose proved slightly toxic to these cells. Non-cytotoxic concentrations of the reagents were used to monitor their effect on cytomegalovirus infection. In MRC5 cells, even the low dose of 4 U/ml myeloperoxidase plus glucose oxidase inhibited synthesis of cytomegalovirus early antigens, as tested by immunofluorescence. Viral release in the supernatant was decreased by 4 logs. In THP1 cells, which produce endogenously hydrogen peroxide, myeloperoxidase alone (8 U/ml) decreased the formation of early and late antigens by 53 and 44%, respectively.