The infection of human peripheral blood leukocytes by varicella-zoster virus (VZV) was studied using an infectious center assay, indirect immunofluorescence and electron microscopy. Subsets of freshly isolated leukocytes were prepared, including granulocytes, mononuclear cells from Ficoll-Hypaque gradients, lymphocytes, and glass-adherent monocytes. When each of these populations was inoculated with VZV (MOI = 0.1), there was no evidence of effective infection. However, when monocytes were cultured in vitro for 7 days, they differentiated into macrophages that were productively infected with VZV. Peak percentages of infectious macrophages were detected 8-24 h after inoculation (mean 17.5%; range 10.2-30.4%). Using indirect immunofluorescence, viral antigens were detected in the cytoplasm and at the nuclear membranes of infected macrophages between 24 and 72 h after infection. Electron microscopy demonstrated the appearance of viral particles in the nucleus by 24 h. Large numbers of virions, often collected in tubules or vacuoles, were present in the cytoplasm at 48 h. The difference between the infection of fresh monocytes and cultured macrophages by VZV might reflect differences in their metabolic or differentiation state. The possible significance of these observations to VZV infection of immunocompromised hosts is discussed.