Immune suppression by regulatory T cells and regulatory B cells is a critical mechanism to limit excess inflammation and autoimmunity. IL-10 is considered the major mediator of B cell–induced immune suppression. We report a novel mechanism for immune suppression through adenosine generation by B cells. We identified a novel population of B cells that expresses CD73 as well as CD39, two ectoenzymes that together catalyze the extracellular dephosphorylation of adenine nucleotides to adenosine. Whereas CD39 expression is common among B cells, CD73 expression is not. Approximately 30–50% of B-1 cells (B220+ CD23−) and IL-10–producing B (B10) cells (B220+ CD5+ CD1d hi) are CD73 hi, depending on mouse strain, whereas few conventional B-2 cells (B220+ CD23+ AA4. 1−) express CD73. In keeping with expression of both CD73 and CD39, we found that CD73+ B cells produce adenosine in the presence of substrate, whereas B-2 cells do not. CD73−/− mice were more susceptible to dextran sulfate sodium salt (DSS)-induced colitis than wild type (WT) mice were, and transfer of CD73+ B cells ameliorated the severity of colitis, suggesting that B cell CD73/CD39/adenosine can modulate DSS-induced colitis. IL-10 production by B cells is not affected by CD73 deficiency. Interestingly, adenosine generation by IL-10−/− B cells is impaired because of reduced expression of CD73, indicating an unexpected connection between IL-10 and adenosine and suggesting caution in interpreting the results of studies with IL-10−/− cells. Our findings demonstrate a novel regulatory role of B cells on colitis through adenosine generation in an IL-10–independent manner.