Interleukin-12 (IL-12) is a crucial cytokine regulating cell-mediated immunity, and may contribute to the development of graft-versus-host disease (GVHD). We investigated serum IL-12 concentrations, interferon gamma (IFN-gamma) production by peripheral blood lymphocytes (PBL) from allogeneic stem cell recipients after IL-12 plus anti-CD3 monoclonal antibody (mAb) stimulation. We also investigated IL-12 production by peripheral macrophages (Mphi) after lipopolysaccharide (LPS) stimulation from allogeneic stem cell recipients and patients receiving donor leukocyte transfusions (DLT) for treatment or prophylaxis of leukemia relapse and Epstein-Barr virus (EBV) lymphoproliferative disease (LPD). PBL from acute GVHD patients produced high IFN-gamma levels after IL-12 plus anti-CD3 mAb stimulation, whereas PBL from patients without acute GVHD produced low levels of IFN-gamma. However, serum IL-12 concentrations were low in both groups. Peripheral Mphi IL-12 production increased in patients who developed acute GVHD compared to patients without acute GVHD. Five patients receiving DLT for treatment or prophylaxis of leukemia relapse developed acute GVHD. IFN-gamma production by PBL stimulated by IL-12 plus anti-CD3 mAb increased, while IL-12 production by peripheral Mphi stimulated by LPS was very high after the development of acute GVHD. However, serum IL-12 concentration remained low. Three patients receiving DLT for EBV-LPD did not develop acute GVHD with no increase of IFN-gamma and IL-12 production. These results indicate that IL-12 may play an important role in the development of acute GVHD after allogeneic stem cell grafting or DLT, and increased IL-12 production by Mphi occurs with various stimuli, including LPS.