Immune regulation produced by B cells has been attributed to production and secretion of interleukin (IL)-10, which is a characteristic of mouse B1 cells. In view of the widespread clinical use of B-cell depletion therapies in autoimmune and malignant diseases, it is important to monitor the function and fate of regulatory B cells. However, there is no consensus regarding the phenotypic identity of human IL-10(+) B cells. Here we show that human CD11b(+) B1 cells, one of two recently described subpopulations of B1 cells, spontaneously produce IL-10 and suppress T-cell activation. In view of the capacity of these B cells to either stimulate T-cell proliferation or suppress T-cell activation, CD11b(+) B1 cells are considered to be capable of orchestrating elements of immune responsiveness and thus are termed "orchestrator B1 cells," or "B1orc," whereas CD11b(-) B1 cells that primarily secrete antibody are termed "secretor B1 cells," or "B1sec."