Primary infection with Epstein-Barr virus (EBV) may arise as infectious mononucleosis (IM) in adolescents and young adults. Morphologically, IM- affected lymphoid tissue is characterized by expanded interfollicular areas with formation of atypical lymphoid blasts. It is assumed that morphology and clinical presentation of IM are related to characteristic patterns of cytokine production by EBV-infected and reactive cells. We studied IM tonsils of eight patients and six normal tonsils with a double in situ hybridization procedure using [35S]- labeled RNA probes specific for various cytokines and digoxigenin- labeled probes for the detection of the nuclear EBV encoded RNA transcripts, EBER 1 and 2. All of the IM cases displayed the same distinct cytokine gene expression pattern. When compared with interfollicular areas of normal tonsils, expression of lymphotoxin (LT), tumor necrosis factor-alpha (TNF-alpha), interleukin-6 (IL-6), and IL-1 beta, but not IL-8 or IL-1 alpha was strongly enhanced in interfollicular areas in IM tonsils. LT was expressed predominantly by EBV-infected cells. TNF-alpha transcripts were also present in EBV- infected cells, although in smaller proportions. IL-6 specific signals were only found in few EBV-infected cells. IL-1 alpha-, IL-1 beta-, and IL-8-specific signals were not observed in EBV-infected cells, but were present at high signal intensity in many cells within and around foci of EBV-infected cells (IL-1 beta), next to areas of necrosis (IL-8, IL- 1 beta), or in epithelial cells (IL-1 alpha). These data suggest that EBV infection in form of IM results in induction of specific sets of cytokine genes in EBV-infected and in neighboring EBV-negative cells contributing to the characteristic morphology and cellular arrangement of the lesion as well as the clinical presentation.