B cells are divided into two categories: conventional or B-2B cells and B-1B cells, the latter of which are distinguished by their different ontogeny. B cell lymphoma 1 (BCL1), the first-reported case of a spontaneously developed mouse B-lymphoma, expresses CD5, surface IgM, Mac-1, CD43 and low level of B220, and is likely to have B-1a cell origin. However, antigens recognized by IgM produced by the BCL1 cells (BCL1-IgM) have not been identified. Here, we demonstrate that BCL1-IgM reacts with Escherichia coli (E. coli). Our initial finding that several recombinant proteins expressed in E. coli bound to BCL1-B20 prompted us to examine the possibility that BCL1 cells may bind E. coli. Indeed, BCL1 cells bound fluorescein-labeled E. coli. To elucidate the structure on the BCL1 cells responsible for E. coli-binding, we produced a monoclonal antibody capable of inhibiting BCL1 binding to E. coli. The antibody recognizes an idiotypic epitope on the BCL1-IgM. Moreover, polyclonal antibody against IgM and secreted BCL1-IgM purified from the supernatants inhibited BCL1 binding to E. coli. Finally, transfection of non-lymphoid cells with cDNA of heavy and light chains of BCL1-IgM conferred the cells ability to bind E. coli. These results clearly indicate that BCL1-IgM bind E. coli and suggest that BCL1 lymphoma is a typical B-1 cell-derived lymphoma, characterized not only by the surface phenotype, but also by the reactivity of its IgM with commensal bacteria.