The potential of recombinant glycosylated human interleukin-6 (rhIL-6) for enhancing immunohematopoietic reconstitution and survival after syngeneic and semiallogeneic bone marrow transplantation (BMT) in BALB/c mice subjected to total body irradiation (TBI) was investigated. rhIL-6 produced enhanced reconstitution of white blood cells as assessed on days 8 and 14 after syngeneic BMT and of platelets as assessed on day 10. Moreover, rhIL-6 treatment produced significant improvement of survival in lethally irradiated mice receiving either syngeneic or semiallogeneic BMT with limiting number of BM cells. This effect of IL-6 was not seen with large BM cell inocula producing high survival by themselves. rhIL-6 showed no toxic effects and did not affect the survival of mice that were lethally irradiated but not reconstituted by BM cells. However, the sensitivity of mice to sublethal irradiation was increased by rhIL-6 in the absence of BM cell transplantation. In experimental conditions inducing graft-versus-host disease (GVHD), in which lethally irradiated (BALB/c x C57BL/6)F1 mice received mixtures of BM and spleen cells from C57BL/6 donors, rhIL-6 was found to enhance GVHD manifestations. No consistent enhancement of T-cell in vitro proliferative responses to allogeneic spleen cells or T- and B-cell-dependent mitogens were seen in the splenocytes obtained from recipients of syngeneic or semiallogeneic BMT. Our data suggest that rhIL-6 may be useful in BMT procedures to enhance thrombopoiesis and hematologic recovery, as well as to increase overall survival rates. In addition, the potentiation of GVHD, which is considered to correlate with graft-versus-leukemia effects, may be of interest in enhancing GVHD-dependent antitumor effects in protocols combining radiochemotherapy with BMT.