Summary. We evaluated 27 factors for their influence on acute graft-versus-host disease (GVHD) in 40 recipients of HLA-identical sibling marrow transplants. These factors included the doses of mononuclear cell subpopulations present in the donor marrow inoculum quantitated using a panel of monoclonal antibodies. Female donors were associated with increased severity of acute GVHD, and the older the female donor the greater this effect. Increasing donor parity was also associated with an increased risk of acute GVHD. The number of T cells, T cells subsets, natural killer cells and monocytes infused did not influence the incidence or severity of acute GVHD in this study, and we could not explain the influence of female donors and of female donor age on acute GVHD by the cellular content of their marrow inocula. We postulate that non-HLA histocompatibility antigen disparity is a more important determinant for acute GVHD than the number of infused donor T cells, especially when female donors are used. The association between acute GVHD and increasing parity suggests that some female marrow donors have been pre-sensitized to their respective recipients by preceding pregnancies.