SUMMARYWe identified three patients (two of them relatives) with RA and signs of scleroderma whose sera contained a high titre of IgG class antibodies against the nucleoli and the nucleoplasm of cells of different mammalian origins. Sera from these patients uniformly immunoprecipitated four polypeptides, from a 35S-methionine-labelled HeLa cell extract, whose mol. wts were 120, 105, 95 and 42 kD. Of these, the 95-kD protein was highly phosphorylated. By immunoblotting, these sera reacted with 105-, 95- and 42-kD proteins and affinity-purified antibodies from these, demonstrating that 105- and 95-kD proteins shared cross-reactive epitopes. Moreover, affinity-purified antibodies from each of these proteins immunoprecipitated the whole complex. Localization studies using immunoelectron microscopy and in vivo actinomycin-D-treated cells demonstrated that the 105-, 95- and 42-kD proteins were present in the granular component of the nucleolus and the nucleoplasm. In addition, the 105- and 95-kD were present in free polyribosomes as well as ribosomes attached to endoplasmic reticulum. Pulse/chase experiments strongly suggested that the complex was accomplished shortly after a 10-min pulse. It was preferentially present in the nucleus after a 2 h chase and in both nucleus and cytoplasm after a 5 h chase. We conclude that a protein complex with a main nucleolar distribution is a new autoantigen (p105-p42) recognized by autoantibodies present in the serum of a subgroup of patients with RA and scleroderma signs. These antibodies could be useful as diagnostic markers and as tools for further studies involving the biology of the nucleolus.