Donor T cell engraftment is increased and occurs more rapidly in mice producing host immune-derived IL-12. Radiation chimeras (B6 or BA and IL-12p40 KO) were conditioned with 9 Gy irradiation and transplanted with 5 × 106 FVB TCD BM cells along with 3 × 105 MACS-purified luc + FVB T-cells. A syngeneic transplant was also performed using non-radiation chimera FVB mice as recipients. (A) In-vivo bioluminescent images of mice were taken weekly beginning 7 days post transplant and ending 42 days post transplant. The color bar represents the scale used for the pseudocolor image. Images are representative from one of two independent experiments of 4–5 mice per group (3 mice per group, syngeneic). (B) Total signal in the region of interest (ROI) (photons/sec) was determined for the whole-body on days 7, 13, 21, 28, 35, and 42 post-transplant. ROI data has been log transformed, however statistical analyses were completed on raw data. Data are combined from two independent experiments of 4–5 mice per group (3 mice per group, syngeneic); *, p < 0.05 comparing WT vs. IL-12p40 KO on day 14 post-transplant; p = 0.092 and p = 0.098 comparing WT vs. IL-12p40 KO on days 28 and 35 post transplant, respectively.