The activity of natural killer cells was found to be deficient in 10 of 12 males with X-linked lymphoproliferative syndrome, a life-threatening proliferation of lymphocytes after infection by Epstein-Barr virus. The activity levels of natural killer cells from affected males were increased after treatment with interferon in vitro, but normal levels of killing were not obtained. Deficient activity of killer cells in individuals with immunodeficiency and chronic infection by Epstein-Barr virus may contribute to the development of lymphoproliferative disorders.