IL-10, a cytokine with pleiotropic functions is produced by many different cells. Although IL-10 may be crucial for initiating protective Th2 responses to helminth infection, it may also function as a suppressive cytokine preventing immune pathology or even contributing to helminth-induced immune evasion. Here, we show that B cells and T cells produce IL-10 during murine Litomosoides sigmodontis infection. IL-10-deficient mice produced increased amounts of L. sigmodontis- specific IFN-gamma and IL-13 suggesting a suppressive role for IL-10 in the initiation of the T-cell response to infection. Using cell type- specific IL-10-deficient mice, we dissected different functions of T-cell- and B-cell-derived IL-10. Litomosoides sigmodontis-specific IFN- gamma, IL-5, and IL-13 production increased in the absence of T-cell- derived IL-10 at early and late time points of infection. In contrast, B-cell-specific IL-10 deficiency did not lead to significant changes in L. sigmodontis-specific cytokine production compared to WT mice. Our results suggest that the initiation of Ag-specific cellular responses during L. sigmodontis infection is suppressed by T-cell-derived IL-10 and not by B-cell-derived IL-10.