Two IgM, kappa anti-myeloperoxidase (MPO) monoclonal antibodies, 6D6 and 9B5, bound to MPO in a solid-phase enzyme-linked immunosorbent assay were derived from the splenocytes of (NZB x NZW) F1 and MRL/lpr-lpr mice, respectively. 6D6 gave a characteristic perinuclear immunofluorescence staining pattern on ethanol-fixed human neutrophils, bound to the native form of MPO by immunoblotting and had a high constant affinity for MPO as demonstrated by real-time specific interaction. 9B5 produced a cytoplasmic immunofluorescence staining pattern, reacted with the heavy chain of MPO and had a low constant affinity for MPO. The heavy-and light-chain variable region genes of monoclonal antibodies (mAb) 6D6 and 9B5 were sequenced and found to be highly homologous to germline genes and to contain negatively charged amino acids in the complementarity determining regions. IgM MPO-binding activity was observed in most BW and MRL/lpr-lpr mouse sera, which may correspond to polyclonal activation of B cells, whereas IgG anti-MPO antibodies could be rarely detected. Thus, this study indicates that (i) BW and MRL/lpr-lpr mice do not delete IgM anti-MPO secreting B cells, do not maintain these B cells in a state of anergy, but most individuals are not able to spontaneously induce the class-switching of this autoantibody population; (ii) IgM anti-MPO antibodies can recognize different epitopes on MPO and produce different immunofluorescence staining pattern on ethanol-fixed human neutrophils, as demonstrated by the immunochemical properties of the two lupus-mouse derived mAb.