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Regulatory B (Breg) cells have been shown to play a critical role in immune homeostasis and in autoimmunity models. We have recently demonstrated that combined anti-T cell immunoglobulin domain and mucin domain-1 and anti-CD45RB antibody treatment results in tolerance to full MHC-mismatched islet allografts in mice by generating Breg cells that are necessary for tolerance. Breg cells are antigen-specific and are capable of transferring tolerance to untreated, transplanted animals. Here, we demonstrate that adoptively transferred Breg cells require the presence of regulatory T (Treg) cells to establish tolerance, and that adoptive transfer of Breg cells increases the number of Treg cells. Interaction with Breg cells in vivo induces significantly more Foxp3 expression in CD4(+) CD25(-) T cells than with naive B cells. We also show that Breg cells express the TGF-β associated latency-associated peptide and that Breg-cell mediated graft prolongation post-adoptive transfer is abrogated by neutralization of TGF-β activity. Breg cells, like Treg cells, demonstrate preferential expression of both C-C chemokine receptor 6 and CXCR3. Collectively, these findings suggest that in this model of antibody-induced transplantation tolerance, Breg cells promote graft survival by promoting Treg-cell development, possibly via TGF-β production.