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Epstein-Barr virus (EBV) infection in infectious mononucleosis (IM) is associated with lymphocyte activation leading to the expansion of cells expressing activation-associated antigens. Most of these antigens are released as soluble molecules in vitro and in vivo.We investigated the serum levels of the soluble forms of the CD8 (sCD8), p55-IL-2R alpha (sIL-2R alpha), and CD30 (sCD30) molecules in 55 patients following primary EBV infection. These data were compared with the phenotypic pattern of circulating lymphoid subsets.In all cases at presentation, lymphocytosis, mainly characterized by the expansion of a CD8+, HLA-DR+, p75-IL-2R beta+, p55-IL-2R alpha- population, was associated with high levels of the investigated soluble molecules. Their mean values (+/- SD) were: 17,172 +/- 12,885 U/mL for sCD8 (vs 334 +/- 95 in controls), 2,922 +/- 2,813 U/mL for sIL-2R alpha (vs 331 +/- 115 in controls), and 477 +/- 451 U/mL for sCD30 (vs 4.9 +/- 6.4 in controls). Follow-up study (15 cases, up to 60 days) showed a progressive decline of all soluble molecules, associated with a reduction of activated CD8+/HLA-DR+/p75-IL-2R beta+ T-cells. By the 30th day, values of sIL-2R alpha and sCD30 (729 +/- 333 U/mL and 20 +/- 21 U/mL, respectively) were only slightly higher than in normal controls, whereas sCD8 levels remained consistently higher (1,777 +/- 1,385 U/mL, p.001).sCD8, sIL-2Ra and sCD30 serum levels in IM reflect the total bulk and/or the activation-related events of infected and reactive cells. The variations in these soluble molecules during the follow-up provide useful information on the in vivo biological modifications occurring after EBV infection.