Graft-vs-Host disease (GVHD) remains a devastating problem in human bone marrow transplantation (1, 2). Because removal of Thy-bearing cells from the donor inoculum has prevented GVHD in murine models (3, 4), it has been hoped that a similar cell surface antigen or combination of antigens could be found in humans. Unfortunately, treatment of human donor cells with various T cell antisera has not yet been successful in preventing GVHD (5). Encouraging results have been reported in five patients who received bone marrow depleted of T cells by the sequential use of soybean agglutinin and the differential sedimentation of cells forming rosettes with sheep red blood cells (6). Although donor T cells are thought to be necessary for initiating GVHD, the immunopathogenesis of GVHD is still not understood. Because donors and recipients are routinely major histocompatibility complex matched and chosen to be nonreactive in mixed lymphocyte cultures human GVHD is thought to result from minor histocompatibility antigen disparities. Lopez and coworkers (7, 8) found a strong association between the incidence of human GVHD and the pretransplant levels of natural killer (NK) activity of the recipients; when the recipient NK activity was low, GVHD rarely developed. They speculated that the NK cell lineage is serving as an important stimulator-inducer. We therefore examined the in vivo effects of anti-asialo GM1 on a murine model of GVHD based on minor antigen disparity. This antiserum has several immunologic effects, including a profound NK suppression. We found that the mice treated with this antibody have normal survival rates, even though they do develop histologic GVHD in the skin. This finding suggests the possibility of a new prophylactic approach to human GVHD and raises many questions regarding the function of asialo GM1-bearing cells in immune regulation.