AbstractObjective. To characterize human autoantigen‐antibody systems related to the mitotic poles and spindles.Methods. Thirty‐seven human sera with autoantibodies staining mitotic poles and spindles in indirect immunofluorescence (IIF) studies were further characterized by immunofluorescence on mitotic cells and by immunoblotting and immunoprecipitation. Clinical diagnoses meeting the American College of Rheumatology criteria were based on chart review and interview with the corresponding physicians.Results. Two autoantibody systems reactive with mitotic poles and spindles were defined. Type 1 nuclear mitotic apparatus (NuMA‐1) antibodies were identified in the serum of 30 patients. Interphase cells showed a fine, speckled, nuclear staining, while mitotic cells had bright staining of the rim of the centrosomes and light staining of the spindles proximal to the centrosomes. In telophase, the staining shifted from the centrosomes to the reforming nuclei. On immunoblotting, anti—NuMA‐1 sera reacted with a 210‐kd protein. The reactivity of these sera was identified (with the aid of reference antibodies) as the previously described NuMA antigen—antibody system. Clinical information was available for only 17 of the 30 patients with anti—NuMA‐1; of these, 17 (53%) had clinical and lip biopsy findings that met the criteria for Sjögren's syndrome. NuMA‐2 antibodies were found in the sera of 7 patients. Interphase cells showed no nuclear or cytoplasmic staining, but mitotic cells had brightly stained poles and spindles. At anaphase/telophase, staining shifted to the midbody and the intercellular bridge. Anti—NuMA‐2 sera immunoprecipitated a protein of 116 kd. This group of patients was more heterogeneous and had both systemic and organ‐specific autoimmune diseases.Conclusion. NuMA protein (here called NuMA‐1) and a 116‐kd protein (here called NuMA‐2) are the major targets of the autoimmune response in the mitotic apparatus, since most of the selected sera (based on IIF staining of the mitotic spindles and poles) recognized 1 of these 2 antigens.