A hybridoma secreting a mouse monoclonal IgG specific for human leukocyte interferon was obtained following fusion of a nonsecreting variant of the myeloma P3X63 Ag8 with splenic lymphocytes from a mouse immunized against interferon incorporated into liposomes. The antibody also recognizes human interferon made by a bacterial recombinant and a fraction of interferon made by lymphoblastoid Namalva cells. Mass production of the antibody was achieved using ascitic growth of the hybridoma in mice.