Studies of a mouse model of genetic resistance to herpes simplex virus type 1 (HSV-1) indicate that the marrow-dependent effector cell of allogeneic resistance plays an important role in natural resistance to this virus infection. Since the marrow-dependent effector cell appears to be closely related to the natural killer (NK) cells, an NK assay with HSV-1-infected fibroblasts [NK(HSV-1)] has been developed to study this resistance mechanism in humans. Incubation of effector and target cells for 12 to 14 h gave the greatest percent specific release (%SR) and kept spontaneous (51)Cr release from infected target cells below 35%. Patients with Bruton's agammaglobulinemia demonstrated significant kill indicating antiviral antibody was not necessary. Seropositive individuals gave a 9% greater%SR than seronegative individuals. Depletion of B-cells consistently diminished NK (HSV-1) for seropositive individuals and augmented kill for seronegative individuals. Although antiviral antibody produced in culture may contribute to NK (HSV-1), depletion of B-cells allowed quantitation of NK (HSV-1) to the exclusion of most of the antibody-dependent kill. The NK cells detected by this assay showed many of the properties reported for NK cells with K562 targets. Two patients with severe herpesvirus infections demonstrated NK (HSV-1) responses greater than 2 standard deviations below the normal mean. Since normal individuals with virus infections have higher rather than lower natural kill, the low NK (HSV-1) may reflect their susceptibility to the virus infection.