Epstein-Barr (EB) virus serology was correlated with the results of immunological investigations of three inherited immunodeficiency diseases, in an attempt to understand the immune mechanisms controlling EB virus infection. In nine patients with Wiskott-Aldrich syndrome (WAS), the constant lack of anti-EB virus associated nuclear antigen (EBNA) was accompanied by a consistent impairment of allogeneic cytotoxicity. We confirmed a frequent absence of anti-EBNA antibody in ataxia telangiectasia (AT), and we showed a correlation between the level of anti-EBNA response and the mixed leucocyte response (MLR), i.e., an absence of anti-EBNA antibody correlated with a decreased MLR. In two of three untreated patients with Chediak-Higashi syndrome (CHS), high persistent titres of anti-EA antibodies were observed, which were possibly related to a defective natural killer (NK) cell activity. In spite of previous infection with EB virus, none of the 41 patients exhibited clinical signs attributable to the virus, suggesting that residual or compensatory mechanisms must have limited activation of the virus. In patients with AT and WAS these mechanisms may include NK cell activity, which is not depressed in these syndromes, whereas in patients with CHS, they may involve T cell cytotoxicity.