The target specificity of natural killer (NK) cells for either tumor cells or virus-infected cells has been investigated. Lymphocyte clones with the surface phenotype of NK cells (CD3-, CD16+) were obtained by limiting dilution of peripheral blood mononuclear cells stimulated with PHA, Herpes simplex virus type 1 (HSV-1), or Varicella-Zoster antigens. Clones were maintained in media with recombinant interleukin 2 (IL-2). Both NK-sensitive (K562 cells) and NK-resistant (Raji cells) targets were lysed by three cloned lines of NK cells. The ability to lyse NK-resistant target cells was largely lost when the cloned lymphocytes were cultured overnight in the absence of IL-2. Effector cells from all three clones were also capable of specifically lysing HSV-1 infected human fibroblasts in comparison with uninfected fibroblasts. We also showed that lysis of HSV-1 infected targets by NK cloned cells was independent of interferons in the culture system.