A stable hybridoma clone derived by fusion of mouse myeloma cells (cell line Fo) and spleen cells of immunized mice has been isolated which secretes monoclonal antibodies against human fibroblast interferon (interferon-p). The antibody inhibits the antiviral activity of human fibroblast interferon in an antiviral assay using human FS4 fibroblast, reacts immunologically with interferon-p separated by sodium dodecylsulfate/polyacrylamide gel electrophoresis and subsequent transfer to nitrocellulose and adsorbs interferon-p immunologically when bound to CNBr-activated Sepharose. It also inhibits the antiviral activity of human fibroblast interferon-fi from which the sugar moiety has been cleaved off by enzymatic treatment. The antibody is therefore probably directed against the protein moiety of the interferon molecules. Monoclonal antibodies are superior to conventional anti- bodies because of their unique specificity directed against a single antigenic determinant and because of their high titers and constant yields [l -31. In the case of interferon so far only two reports have described the successful isolation