R4-6A2, a rat antimouse IFN-γ MoAb, inhibits the protective effect of IL-12 against GVHD. Lethally irradiated B10 mice received 5 × 10⁶ TCD B10 BMC plus 10 × 10⁶ A/J BMC and 15 × 10⁶ A/J spleen cells with no further treatment (Allo BMT, n = 8), or with IL-12 (Allo BMT + IL-12, n = 8) or R4-6A2 (Allo BMT + R4-6A2, n = 5), or both (Allo BMT + IL-12 + R4-6A2, n = 8). Syngeneic controls received 5 × 10⁶ TCD B10 BMC with treatments of IL-12 and R4-6A2 (SYN BMT + IL-12 + R4-6A2, n = 3). Four thousand nine hundred units IL-12 and 2.5 mg R4-6A2 were administered by i.p. injection on day 0 and day 1 with respect to BMT, respectively.