B-1 cells contribute to susceptibility in experimental infection with Leishmania (Leishmania) chagasi.  The immune response to leishmaniasis is complex, and the result of infection depends on both the genetic composition of the Leishmania species and the immunity of the host. Clinical and experimental evidence suggest that the activation of B cells leads to exacerbation of visceral leishmaniasis. However, the role of B-1 cells (a subtype of B lymphocytes) in the pathogenesis of experimental visceral leishmaniasis has not yet been elucidated. In this study, we investigated the importance of B-1 cells in experimental infection with Leishmania. (L.) chagasi. Our results showed that BALB/XID mice (X-linked immunodeficient mice which are genetically deficient in B-1 cells) infected with L. (L.) chagasi for 45 days had a significant reduction in parasite load in the spleen when compared with control mice. Cytokine analysis showed that the BALB/XID mice had lower amounts of IL-10 in their sera compared with control group. In addition, the transfer of B-1 cells from wild type mice into IL-10KO animals led to an increase in susceptibility to L. (L.) chagasi infection in the IL-10KO mice, suggesting that the IL-10 produced by these cells is important in experimental infection. Our results suggest that B-1 cells may play an important role in susceptibility to L. (L.) chagasi.