A primary function of B lymphocytes is immunoglobulin production; however, the therapeutic benefit of B cell depletion in autoimmune diseases previously thought to be T cell mediated suggests that some B cells fulfill other roles in autoimmunity. We examined the recently identified human B1 cell population for T cell stimulatory activity. We found two kinds of B1 cells that are distinguished by multiple surface markers and distinct transcriptomic profiles. In both umbilical cord and adult peripheral blood, a CD11b(+) subset constitutes ~1 out of every 8-10 B1 cells, whereas a CD11b(-) subset constitutes the remaining B1 cells. These B1 cell populations differ functionally. CD11b(-) B1 cells spontaneously secrete much more IgM than CD11b(+) B1 cells. In contrast, CD11b(+) B1 cells express more CD86, and more efficiently stimulate allogeneic CD4(+) T cell expansion, than CD11b(-) B1 cells. The frequency of these CD11b(+) B1 cells is markedly elevated in lupus patients. CD11b(+) B1 cells in lupus patients express more CD86 and have increased T cell-stimulating activity in disease. This work distinguishes a novel, T cell-interacting B1 cell population whose abundance and activity may be a reflection of, and a therapeutic target in, autoimmune disease.