The 85Sr retention of syngeneic and allogeneic tibia grafts taken from 16-day-old mice was investigated using recipients of 16 days, 12 weeks and 1 year of age. 85Sr retentions of syngeneic grafts of the tibia were generally higher than those of allogeneic ones. Additionally, there was in all grafts an increased 85Sr uptake with increasing age of the recipients. Simultaneously an increased excretion of 85Sr was observed in the older recipients. This finding may be explained by the decreasing uptake of 85Sr by the whole skeleton of the older recipients, which was recorded by the decreasing 85Sr uptake of recipient tibiae, leaving more 85Sr in the common pool to be taken up by the grafts or to be excreted. However, the proportion of 85Sr uptake of syngeneic and allogeneic grafts was also changed with age of the recipients, leading to a decrease of the ratio 85Sr in syngeneic grafts versus 85Sr in allogeneic grafts of older recipients. This points to a decrease of the immunological response with increasing age of the recipients.