The role of complement receptors in phagocytosis of herpes simplex virus (HSV) by PMN was examined. Complement components were deposited on the surface of the virus particle in the presence or absence of specific anti-HSV antibodies. Flow cytometry was used to analyze the phagocytosis of fluorescence-labeled viruses and demonstrated that although a virion is able to associate with PMN in the presence of complement alone, the granulocyte is not triggered to mount a metabolic burst. Efficient stimulation of PMN occurs when complexes are formed consisting of virus, specific antibodies, and complement. To address the question whether the viruses were inside or outside the cell, a combined enhancement/quenching method was developed using ammonium chloride as a lysosomotropic agent and trypan blue as a quenching dye. The data indicate that Fc receptor-mediated phagocytosis by PMN results in the ingestion of all cell-associated herpes virions. Interactions of virions through PMN-complement receptors CR1 and CR3 results solely in binding to the PMN but not in internalization. Interactions via both complement and Fc receptors cause synergistic stimulation of the PMN and result in very efficient association of viruses, greater than 80% of which were inside the cell.