Tularemia was diagnosed in 57 patients during an outbreak in central Norway in 1984 and 1985. Clinical categories of the disease showed seasonal variations. A bacterial microagglutination test and an enzyme-linked immunosorbent assay (ELISA) with class-specific antibodies against Francisella tularensis outer membrane (OM) antigens were evaluated for the early diagnosis of tularemia. ELISA with immunoglobulin G (IgG), IgA, or IgM antibodies and the microagglutination test differed only marginally in diagnostic sensitivity. The OM preparation harbored F. tularensis agglutinogens and contained a variety of proteins, several of which functioned as immunogens in tularemia patients, as shown by Western blotting (immunoblotting). All 12 patients tested produced antibodies against a 43,000-molecular-weight OM protein. Individual variation was noted with regard to antibody response against other OM antigens. The OM is a suitable antigen preparation in ELISA for the diagnosis of tularemia and, presumably, contains antigens important in the immunobiology of tularemia.