A test method that provides data useful in predicting the probability of onset of acute graft-versus-host disease (GVHD) is described along with a kit for performing the method, and a pharmaceutical preparation and a molecular targeted therapy for treating or preventing GVHD. The test method includes measuring the blood DNAM-1 concentration of a patient of hematopoietic stem cell transplantation from bone marrow or the like over a period after the transplantation to provide data concerning the transition of the concentration to an abnormally high level deviating from the normal range, whereby the probability of the development of acute graft-versus-host disease is predicted, the risk of the development is estimated, or therapeutic effects after the development are evaluated. Concerning the molecular targeted therapy and pharmaceutical preparation used therefor wherein blood DNAM-1 of a GVHD patient or a graft recipient that is a possible patient is used as a target molecule, GVHD is treated or prevented by administering an anti-DNAM-1 antibody that is a neutralizing antibody.