HLA matched activated lymphocytes in mononuclear cells separated from peripheral blood or umbilical cord blood are proliferated and activated. After separating and collecting, the HLA matched activated lymphocytes are employed as the main component of a composition for promoting survival of transplanted hematopoietic stem cells. The obtained composition is widely usable in, for instance, prevention of survival failure of transplanted hematopoietic stem cells and therapy for promoting the survival thereof. Although the dose of the composition varies depending on the age, conditions, etc. of a patient, a humanized antibody is administered in a dose of from 0.2 to 20 ml/kg/day to mammals including humans. The composition is administered by intravenous injection either once a day (single administration or continuous administration) or intermittently once to 3 times in a week or once in 2 or 3 weeks.
