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International audience; A rabbit serum which had previously been reported to have an immunological affinity for centrosomes of human cell lines was shown also to be specific for the nucleus. Optical and ultrastructural immunolocalization in HeLa cells showed that this specificity is restricted to the fibrillar centre of nucleoli either in untreated or actinomycin D treated interphase cells. In mitotic cells discrete labelling was observed on chromosomes and shown to correspond, on spread metaphase plates, to the short arms of acrocentric chromosomes, i.e. to the nucleolar organizer regions (NORs). Using independent cell fractionation procedures in the human T-lymphoblastic KE 37 cell line and purification of immunoglobulins by affinity to antigens detected by electrophoresis and blotting, a strict correlation between immunoreactive proteins and cytological staining was established. The nucleolar specificity was shown to correspond to a protein with an Mr of 80,000 while the centrosomal specificity corresponded principally to a protein doublet of 60,000-65,000. These antigens share common epitopes as shown by the staining of both NOR and centrosome by immunoglobulins purified by affinity to either type of protein.