: Infection with the trematode helminth S. mansoni affects more than 200 million people worldwide. Infected patients are thought to show a decreased incidence of asthma and autoimmune diseases, which is, among others, considered a result of an increased production of the immunoregulatory cytokine IL-10. However, the location and the type of cell that is responsible for the highest production of IL-10 in vivo is still unknown. Here, we describe the use of the IL-10 reporter mouse TIGER for the study of murine schistosomiasis and introduce a novel tool, which we have called the TIGER index (TI). This index combines data from flow cytometric measurements and cell count analysis and allows identifying the cell type with the highest contribution of IL-10 during the course of infection in the secondary lymphoid organs, sites of extensive immunoregulatory activity in schistosomiasis. The strength of the TI is the possibility to analyze IL-10 production in a long term experiment without the need of an external standard between each time point of analysis. In this paper we have calculated the TI for the mesenteric lymph nodes and the spleen in the course of a chronic infection with S. mansoni. Using the TI, we identified CD4(pos) CD25(pos) and CD4(pos) CD25(neg) cell populations as the highest producers of IL-10 in the mesenteric lymph node and the spleen in chronic schistosomiasis, respectively, whereas B cells, NK cells and NKT cells showed a lower contribution to IL-10 production throughout the infection. The TI is thus a highly useful tool to measure the relative contribution of different cell types, which are responsible for the in vivo production of IL-10 in the secondary lymphoid organs during the infection with S. mansoni.