A retrospective analysis of clinical laboratory findings in 62 patients with persistent relapsing lymphadenopathy of obscure etiology was carried out. Virus-specific markers of active infection (IgM-VCA, IgG-EA) or a previous infection caused by Epstein-Barr virus (EBV) (EgG-EBNA-1) were detected in the serum of 57 patients. Positive staining of histological preparations of biopsy specimens of the lymph nodes by EBV-specific antibodies to EBNA-2 indicated the fact of EBV infection but did not characterize the infection phase. A criteria for evaluating the activity of EBV infection during other than the active period is suggested. An inverse relationship between the level of beta-globulins and level of IgG-EBNA-1 antibodies was detected, which can serve as a marker of immunodeficiency in EBV infection.