Human endothelial cells or human foreskin fibroblasts infected with herpes simplex viruses (HSVs) potently inhibit the lytic activity of natural killer cells and interleukin 2-activated killer cells. The inhibition occurs after as little as 8 hr of viral infection and requires contact between effector cells and HSV-infected targets. Inhibition evidently stems from direct blockade of killer cell function because killer cells placed atop HSV-infected targets rapidly become incapable of lysing subsequently added HL-60 or K-562 cells. The impairment of killer cell function is prevented when protein glycosylation in HSV-infected cells is blocked with tunicamycin. These studies may be relevant for understanding the persistence of herpes simplex virus infections and, further, suggest a mechanism for failed immune surveillance.