B1a B cells have been implicated in the pathogenesis of systemic lupus erythematosus; however, their precise contribution to the disease remains unclear. Here we analysed isotype expression, organ accumulation and autoreactivity of B1a cells in the NZB/W F1 murine model for systemic lupus erythematosus using flow cytometry, ELISPOT and adoptive cell transfer. In the course of lupus, the B1a compartment is expanded and B1a cells class switch to IgG. Class-switched B1a B cells were excluded from the peritoneal cavity but accumulated in the spleen and target organs such as kidneys and thymus. Autoreactive B1a B cells preferentially class switch, which leads to a subsequent loss of autoreactive B1a cells from the peritoneal cavity. We propose a model whereby autoreactive B1a B cells become activated early in the course of lupus and class switch to IgG. These autoreactive B1a B cells accumulate in the spleen and affected organs where they presumably secrete autoantibodies and contribute to the local and systemic pathogenesis.