Graft-versus-host disease (GVHD) is a major obstacle to successful bone marrow transplantation. The role of interferon (IFN) in GVHD is currently unclear. We have previously shown that interferon (IFN) is produced in vitro by alloantigen-stimulated murine bone marrow cells. This study was initiated to examine whether IFN production occurs in vivo during GVHD. Lethally irradiated mice were given bone marrow (107) and/or spleen cells (2x107) from either allogeneic or syngeneic mice. Mice given allogeneic spleen cells or bone marrow and spleen cells showed signs of GVHD within 10 days and usually died within 20 days of transplantation. Mice undergoing GVHD were found to have significant levels of IFN activity in their sera. Serum IFN was detected early (day 3) after transplantation with optimal IFN activity (~ 80 units) occurring at 5-6 days. The IFN activity present in the sera obtained from mice with GVHD was identified as IFN [alpha]/[beta] by using specific antisera against IFN [alpha]/[beta] and IFN [gamma]. In contrast, irradiated mice given T-cell-depleted allogeneic bone marrow and spleen cells failed to develop GVHD and had no detectable serum IFN activity. Irradiated mice given syngeneic bone marrow and spleen cells or only allogeneic bone marrow cells did not develop GVHD and did not produce detectable IFN activity in their sera. These results show that serum IFN activity correlates well with GVHD and opens for speculation the possibility that IFN may be involved in the pathogenesis associated with GVHD.