Serum autoantibodies produce typical immunofluorescence staining patterns on HEp-2 cells, which are frequently used for diagnostic purposes. These include antibodies recognizing cytoskeletal and nuclear epitopes. The detailed analysis of human monoclonal antibodies (MAbs) should help to understand which antigens or autoantigens were involved in the generation of these immune responses. Here, three MAbs are described staining HEp-2 cells in a characteristic pattern. They were derived from peripheral blood B cells of two patients with rheumatic diseases (rheumatoid arthritis and relapsing polychondritis). Their binding reactivities were characterized in detail in several assay systems and their affinities measured. Although the antibodies differed in their fine specificity and crossreactivity, all three MAbs (2 IgM, 1 IgA) bound to purified cytoskeletal antigens (desmin) and, in addition, to cartilage antigens (human collagen type II, proteoglycans). The binding to HEp-2 cells could be inhibited specifically with soluble antigens as shown by intracellular flow cytometry. The affinities for both groups of antigens were relatively high (examples: K(D) (desmin) = 0.1 x 10(-7) M; K(D) (collagen) = 3.5 x 10(-7) M). Two of the MAbs also bound to heat-shock protein 60 (HSP60) derived from Mycobacterium tuberculosis. The results prove that antibodies and B cells with reactivity to both intracellular cytoskeletal and nuclear antigens and exogenous antigens (e. g. HSP60) exist in patients with rheumatic diseases. Similar to an animal model such human B cells may be induced by the exogenous antigen (HSP60) and crossreact with local auto-antigens related to the disease (cartilage). In this way they might contribute to pathogenic processes. Due to their additional crossreactivity with intracellular cytoskeletal and nuclear antigens, these antibodies simultaneously can be detected in the HEp-2 immunofluorescence assay.