Mothers of children with neonatal lupus erythematosus (NLE) and heart block, as well as patients with Sjögren's syndrome (SS) and systemic lupus erythematosus, have serum autoantibodies that recognize SS-A/Ro autoantigens including the 60-kDa ribonucleoprotein. By yeast 2-hybrid screening, we identified a novel 75-kDa protein (pp75) that interacts with the carboxyl 70% of 60-kDa SS-A/Ro. The specificity of interaction was confirmed using mammalian 2-hybrid and chemical crosslinking studies. Immunoprecipitation with radiolabeled HeLa cell extracts showed that pp75 was phosphorylated and associated with 2 other phosphoproteins of 64 kDa. In Northern blot analysis, pp75 was expressed in all tissues analyzed; the highest expression was in the human heart. Based on immunofluorescence of transfected HeLa cells, pp75 is localized predominantly in the cytoplasm, an observation confirmed by immunohistochemistry in untransfected cells. Based on Western blot and ELISA assays, sera from 14 of 84 mothers of children with NLE recognized pp75, including 1 mother in whom anti-SS-A/Ro antibodies were not detected. In addition, sera from 5 of 80 patients with SS were positive for anti-pp75 antibody. Identification of molecular partners is a first step toward elucidating the functions and possible involvement in pathogenesis of long-recognized autoantigens such as 60-kDa SS-A/Ro, which are at present poorly understood.