Th17 cells and Th1 cells coordinate to play a critical role in the formation of inflammatory bowel diseases. To examine how Th17 and Th1 cells are regulated at inflammatory sites, we used Th1-dominant CD4(+)CD45RB(high) T cell-transferred RAG-2(-/-) and Th1/Th17-mixed IL-10(-/-) mice. Interestingly, not only did colitic RAG-2(-/-) mice that were parabiosed with WT mice show significant amelioration of colitis, but amelioration of disease was also observed in those parabiosed with colitic IL-10(-/-) mice. To assess the interference between Th1 and Th17 colitogenic T cells, we co-transferred colitogenic CD4(+) T cells from the lamina propria (LP) of CD4(+)CD45RB(high) T cell-transferred RAG-2(-/-) mice and IL-10(-/-) mice into RAG-2(-/-) mice. Surprisingly, the co-transferred RAG-2(-/-) mice showed a vast cellular infiltration of LP CD4(+) T cells similar to that seen in RAG-2(-/-) mice re-transferred with the cells from colitic RAG-2(-/-) mice alone, but the co-transferred RAG-2(-/-) mice did not have the wasting symptoms, which are also absent in RAG-2(-/-) mice transferred with cells from colitic IL-10(-/-) mice alone. Furthermore, the percentages of Th1 and Th17 cells originating from IL-10(-/-) mice and those of Th1 cells originating from colitic RAG-2(-/-) mice were all significantly decreased in the co-transferred mice as compared with the singly-transferred paired RAG-2(-/-) mice, suggesting that Th1 and Th17 cells are in competition, and that their orchestration results in a merged clinical phenotype of the two types of murine colitis.