A quantitative comparison was made of the metabolism of collagen in functional and non-functional tendon grafts. This was accomplished by the transplantation of ³H-proline-labeled tendon grafts into non-radioactive recipient animals. The experiments were performed as isografts and allografts in rats, and as xenografts between rats and guinea pigs.

1. After three months of implantation as isografts, the functional grafts lost as much of the original collagen as the non-functional grafts, 54 per cent versus 52 per cent, while the analogous percentages in allografts were 63 per cent and 60 per cent, and in xenografts, 99 per cent and 87 per cent.

2. Functional isografts demonstrated greater capability (64 per cent) to synthesize new collagen than the non-functional isografts (12 per cent), and so did allografts, while xenografts gained little to any new collagen.

3. As a result of collagen lost and collagen replaced, the functional isografts were able to maintain their collagen mass while non-functional isografts decreased their collagen mass (-48 per cent) due to decreased synthesis of new collagen. Xenografts lost almost all of their mass due to an inability to produce new collagen after transplantation.

The isotopic experiments demonstrate quantitatively the persistence of tendon collagen mass in the face of massive turnover of tendon collagen. The effect of either non-function or antigenicity is to decrease the collagen mass by means of diminished replacement of destroyed old collagen with newly synthesized collagen.