Host immune-derived IL-12 drives donor-T cells towards a Th1 phenotype. Radiation chimeras (B6 or BA and IL-12p40 KO) were conditioned with 9 Gy irradiation and transplanted with 5 × 106 BA.B10 TCD BM cells and 3 × 105 MACS-purified B10.BR T-cells. Recipient splenocytes were prepared 10 days post transplant and analyzed via flow-cytometry for the percentages of regulatory T cells as well as TNF-α and IL-10 producing donor T-cells. Donor-T cells were gated on using congenic markers (CD45.1, CD45.2, CD90.1) and CD3. Regulatory T cells were defined as T cells expressing both CD4 and FoxP3. Representative flow plots for B6 (A) and IL-12p40 KO (B) radiation chimeras are shown. Percentages of FoxP3+ T cells in the CD4 T cell compartment are shown in (C); *, p < 0.05 comparing WT vs. IL-12p40 KO. Data shown are from two independent experiments of 4–5 mice per group. Percentages TNF-α and IL-10 producing CD4+ donor-T cells are shown in (D) and (E), respectively. IL-10 data are representative from one of two independent experiments with 4–5 mice per group using pre-transplant irradiation of 9 Gy or 10 Gy (9 Gy is shown). TNF-α data is combined from two independent experiments with 4–5 mice per group. **, p < 0.01 comparing WT vs. IL-12p40 KO.