The combined grafting of young bone marrow and newborn thymus performed in old mice was effective in restoring the impaired immune functions, but the same treatment performed in middle-aged adult mice had no effect on the life span of C3H/MTV female mice. Sequential multiple newborn thymus graftings starting at young adult age were effective in enhancing immunological functions, delaying the onset of tumor and extending the survival rate to certain degree in the first half of the experimental course in both C3H/MTV as well as C57BL/6 mice, but these effects were not observed in the latter half of the experimental course. It was suggested that multiple newborn thymuses sequentially implanted into the peritoneal cavity underwent atrophy and these atrophic thymuses had a suppressive effect on the host immune system. In autoimmune prone B/WF1 mice, however, the combined grafting of young bone marrow and newborn thymus resulted in suppression of antibody formation to SRBC, and single grafting of either young bone marrow or newborn thymus resulted in a trend of increase in the antibody formation to SRBC. The sequential multiple newborn thymus graftings in B/WF1 mice brought about aggravation of kidney diseases and shortening of the mean life span. On the contrary, administration of thymosin in B/WF1 mice resulted in amelioration of kidney disease and elongation of the mean life span.