Interferon-gamma (IFN-gamma) is known to be involved in graft rejection of solid organs and acute graft-versus-host disease (GVHD). Its role, and especially that of soluble IFN-gamma receptor (sIFN-gamma R), in bone marrow transplantation (BMT) has not been established. We evaluated the sera of 27 patients following BMT. Fourteen of them underwent uneventful BMT, whereas 13 developed transplant-related complications, including acute GVHD (n = 5), early rejection (n = 4), or relapse of basic disease (n = 4). Soluble IFN-gamma R and IFN levels were evaluated at day -10 (preconditioning), day 0 (day of BMT), day of engraftment, and during BMT-related complications using sIFN-gamma R-specific monoclonal antibodies (McAB) followed by double-sandwich ELISA, and a sensitive radioimmunoassay respectively. In normal controls (n = 80) sIFN-gamma R and IFN-gamma levels in the sera were 0.5 +/- 0.05 and 0.3 +/- 0.04 ng/ml respectively. Soluble IFN-gamma R levels increased in direct correlation with engraftment (0.63 +/- 0.11 ng/ml at the day of BMT to 1.43 +/- 0.16 ng/ml at the day of engraftment; n = 14; P < 0.001). IFN-gamma levels increased in direct correlation with engraftment (0.37 +/- 0.03 ng/ml at the day of BMT to 5.69 +/- 1.64 ng/ml at the day of engraftment; n = 14; P < 0.001). In five patients with GVHD sIFN-gamma R levels increased from 0.43 +/- 0.19 ng/ml at the day of BMT to 1.73 +/- 0.17 ng/ml (P < 0.004) at the time of GVHD. Similarly, IFN-gamma levels increased from 0.43 +/- 0.08 ng/ml at the day of BMT to 3.03 +/- 0.5 ng/ml at the time of GVHD (P < 0.05). Both graft rejection and early relapse were associated with an elevation of IFN-gamma levels. In short, both s-IFN-gamma R and IFN-gamma were found to be significantly elevated during engraftment and GVHD. Hence these cytokines may be used as a tool for assessing engraftment and AGVHD.