There is no suppression of cellular or humoral methylated bovine serum albumin (mBSA)-specific immunity with transfer of Treg cells. Pre-activated CD4+CD25+ cells (1 × 106) were transferred at the time of antigen-induced arthritis (AIA) induction. (a) Delayed-type hypersensitivity (DTH) reactivity against mBSA in vivo was tested 7 days later by an intradermal antigen challenge into the ears. (b) Antigen-specific proliferation ([3H]thymidine incorporation) and (c) cytokine production (ELISPOT) of draining lymph node cells was measured 14 days after AIA induction. (d) Serum levels of IgG against mBSA, collagen type I, collagen type II and cartilage proteoglycans were measured with ELISA after 14 days of AIA. Proliferation, DTH reaction, cytokine production, and serum IgG titres were tested in six animals per group. #P < 0.05 for CD4+CD25+ versus CD4+CD25-; *P < 0.05 for CD4+CD25- versus phosphate-buffered saline.