1) The highest incidence of runt disease is obtained in case of thymus cell transplanted mice among cell graft experiments of various tissues of Db strain mice to R(Ⅲ)strain mice and those of R(Ⅲ)strain mice to Db strain mice.
2) The incidence is found to vary with the age of the donor mice from which thymus cells are supplied. The thymus cell of newborns has no ability to cause runt disease, but the incidence of the disease gradually increase after the age of 2 to 4 weeks of the donor mice. 3) Destruction and loss of lymphocytes are histologically demonstrated in the thymus of the thymus cell transplanted group. The same changes are observed in the spleen of the spleen cell transplanted group. 4) A marked increase of lymphoreticular cells were seen in the course of runt disease. 5) These findings suggest that the thymus of adult mice plays a significant role in immunological response of the body.