It is described the use of thymosin alpha 1 for preparing a medicament useful for the prevention or treatment of graft-versus-host disease or graft rejection reactions in organ transplantation, in a mammal subject, in which the cells, tissues or organs for transplant is selected from the group comprising: stem cells, hematopoietic stem cells, bone marrow, heart, liver, kidney, lung, pancreas, small intestine, cornea or skin.