Splenic IL-10-producing CD1dhighCD5+ B cells have been designated as regulatory B cells (Breg or B10). These rare IL-10- producing B cells represent only 1%-2% of spleen B cells and 7%-8% of peritoneal B cells in mice but are not normally detectable in blood or lymph nodes. B10 cells are a unique subset of potent regulatory B cells different from B-1a, B-1b, or CD21+CD23- mantle zone B cells. B10 cells produce IL-10 and negatively regulate T cell function. BioLegend's Mouse Regulatory B Cell Flow Kit is composed of fluorochrome conjugated anti-mouse CD19, CD5, and CD1d. It is optimized for detection or isolation of B10 cells by flow cytometry.