Fibrillarin, a component of the U3 RNP particle, is a target for the spontaneously arising autoantibodies in human scleroderma and a monoclonal autoantibody (72B9) derived from the autoimmune mouse strain (NZB x NZW) F1. Autoantibodies against fibrillarin can also be induced in H-2s mice by treatment with mercuric chloride (HgCl2). The objective of this study was to compare the spontaneously occurring anti-fibrillarin autoantibody response with the autoantibody response induced by HgCl2 treatment. Immunofluorescence microscopy on human HEp2, mouse 3T3, and Xenopus XIK-2 cells, immunoblotting with use of nuclear extract from human MOLT-4, mouse 3T3, and Xenopus XIK-2 cells, and immunoprecipitation with use of in vitro translation products of RNA transcripts from yeast fibrillarin cDNA were used in this analysis. Both spontaneous and induced autoantibodies displayed common reactivity in that, irrespective of the antigenic source, they gave the same nucleolar immunofluorescence pattern and a restricted immunoblotting reactivity targeting predominantly the 34-kDa protein fibrillarin. Immunoprecipitation of N- and C-terminal truncated fibrillarin constructs also demonstrated a common pattern of reactivity. All Abs precipitated a fragment comprising amino acids 1-312 but not a smaller fragment containing amino acids 1-257. The majority of sera could not precipitate an N-terminal truncated molecule consisting of amino acids 157-327. These immunoprecipitation experiments support recognition of a common epitope requiring both N- and C-regions, which may be exemplified by the reactivity of murine monoclonal anti-fibrillarin autoantibody 72B9. Our results indicate that spontaneous human and toxin-induced murine autoantibodies to fibrillarin share common reactivity against this highly conserved nucleolar protein.