Interleukins have been postulated to exert an important modulatory and recruiting role in the Theiler's murine encephalomyelitis virus (TMEV)-induced demyelination of the central nervous system (CNS) in SJL/J mice. Using bio- and radioimmunoassays, we have detected and quantified some of the interleukins suspected to play a role in this immune-mediated process. The interleukins 1 and 2 (IL-1, IL-2) and interferon-gamma (IFN-gamma) have been measured in homogenates from the CNS and sera from infected animals, as well as in supernatants from antigen-specific in vitro-stimulated spleen and meningeal cells. IL-1 was detected both in CNS tissue homogenates (approximately 20-40 fmol/ml) and in splenic cultures (200 U/ml). IL-2 was detected only after TMEV-specific antigenic stimulation in spleen cultures (approximately 120 U/ml). In vitro, IL-2 and IL-1 synthesis appear in a dose- and time-dependent manner. IFN-gamma could not be found in any case. The precise nature of IL-1 and IL-2 activity was further assessed by HPLC. The above results strongly indicate the presence of functionally active macrophages in the CNS infiltrates of cells triggering this autoaggressive immune process. In addition, we propose a central role for IL-1 in augmenting the intracerebral immune response leading to the inflammatory demyelination induced by TMEV.