IL-10 is a pleiotropic cytokine with stimulatory and inhibitory properties, and is thought to have a protective role in rheumatoid arthritis and collagen-induced arthritis (CIA). In this study, we investigated how IL-10 deficiency affects CIA and anti-collagen type II (CII) Ab-transferred arthritis in C57BL/10. Q (B10. Q) mice. The B10. Q. IL-10−/− mice had an 8-cM 129/Ola fragment around the IL-10 gene. The mice were treated with antibiotics, appeared healthy, and had no colitis. T cells from IL-10−/− mice expressed similar levels of IFN-γ, IL-2, and IL-4 after mitogen stimulation; however, macrophages showed a reduced TNF-α production compared with IL-10+/− littermates. IL-10−/− mice had an increased incidence, and a more severe CIA disease than the IL-10+/− littermates. To study the role of IL-10 in T cell tolerance, IL-10−/− were crossed into mice carrying the immunodominant epitope, CII (256–270), in cartilage (MMC) or in skin (TSC). Both IL-10−/− and IL-10+/− MMC and TSC mice were completely tolerized against CIA, indicating that lack of IL-10 in this context did not break tolerance. To investigate whether IL-10 was important in the effector phase of CIA, arthritis was induced with anti-CII Abs. Surprisingly, IL-10−/− were less susceptible to Ab-transferred arthritis, as only 30% showed signs of disease compared with 90% of the littermates. Therefore, IL-10 seemed to have a protective role in CIA, but seemed to exacerbate the arthritogenicity of anti-CII Abs. These data emphasize the importance of studying IL-10 in a defined genetic context in vivo, to understand its role in a complex disease like arthritis.