The ability of leukemic leukocytes to support the replication of herpes simplex virus (HSV) was studied. Mononuclear leukocytes (MNL) from the peripheral blood of patients with a variety of lymphoid leukemias were isolated on Ficoll-Hypaque gradients and infected with HSV at a multiplicity of infection of 5 to 10. No virus growth was detected in cells from patients with chronic lymphocytic leukemia (9), acute lymphocytic leukemia (1), or lymphosarcoma cell leukemia (2), HSV replication did occur in hairy cell leukemic MNL from all of 4 patients studied. Maximal titers of 10(3.7) to 10(4.7) PFU/ml occurred 1 to 7 days after incubation. By electron microscopy, herpesvirus particles were seen in the nuclei of these infected cells after 3 days of culture, but none was seen in the cells not exposed to virus. Fluorescent antibody examination confirmed the presence of HSV antigens in the nuclei of infected hairy cells. No difference in the adsorption or penetration of the virus was found with the various MNL studied. Productive infection of the cells thus appeared to depend on the ability of the leukocyte ;o support a later stage of infection, either uncoating or replication of the virus.