A preparation of allogeneic HLA-matched leukocytes treated with a derivative of placental protein 14 (PP14) with potential immunomodulating activity. PP14 derivative-treated HLA-matched donor mononuclear cell-enriched leukocytes contain at least fifty-five percent early-apoptotic T cells; after infusion and when processed by recipient dendritic cells, early-apoptotic T cells may help induce a decrease in the donor effector T-cell responses against the recipient of an allogeneic hematopoietic stem cell (HSC) transplant, thereby minimizing allogeneic HSC transplant-related graft-versus-host disease (GVHD). PP14, a 162 amino acid glycosylated protein secreted by the late secretory phase endometrium, binds to T cells in a carbohydrate fashion and has been shown to induce T-cell apoptosis; maternal immune tolerance to the fetus and pregnancy-related remissions of autoimmune disease may involve PP14-induced T cell apoptosis.