To investigate a possible role of interferons (IFNs) in lesion pathogenesis, central nervous system tissue from multiple sclerosis patients and control subjects was stained by immunocytochemical techniques in combination with monoclonal antibodies or polyclonal antisera for the demonstration of IFN-α, IFN-β, and IFN-γ. The results were correlated to lesion activity, to the presence of class I and class II major histocompatibility antigen-positive astrocytes, and to the composition of cellular infiltrates. IFNs were detectable in active but not in inactive chronic multiple sclerosis lesions. In acute multiple sclerosis plaques, all three types of IFN were widely distributed on glial elements and infiltrating cells. In active chronic multiple sclerosis, labeling of cells for IFN-α, IFN-β, and IFN-γ was most pronounced at the lesion edge and displayed distinct distribution patterns. Overall, IFN-γ was more common than IFN-α, IFN-β, and was predominantly found on astrocytes. IFN-α, was detectable mainly on macrophages. Distribution of IFN-β, partially overlapped with that of IFN-γ and IFN-β, in that it was present on some astrocytes and on some macrophages. IFNs were rare in normal white matter remote from lesions and in the gliotic lesion center. La antigen and human leukocyte antigen ABC on astrocytes were distributed similarly to IFN-γ and IFN-β, respectively. These findings indicate that IFN-γ may play a role in active lesion growth in multiple sclerosis, whereas IFN-α and IFN-β may exert some local immunosuppressive effect. IFNs were also detectable in central nervous system tissue from patients with some other neurological diseases but were virtually absent from tissue from normal control subjects.