Tail or Achilles tendons were taken from rats that were repeatedly prelabeled with ³H-proline so as to be in an isotopic steady state. Fresh and frozen isografts and allografts, plus frozen xenografts were implanted as functional Achilles grafts into rats and guinea pigs. At one month of implantation isografts showed a one to one replacement of resorbed old collagen with non-radioactive new collagen without a change in total collagen mass. Allografts had almost complete replacement with a small loss of collagen mass. After three months of implantation, isografts showed continued maintenance of collagen mass while allografts showed a continued decrease in replacement and total mass. The major effect of the immune response on collagen turnover in tendon allografts was to partially inhibit the production of new collagen with a resultant atrophy in collagen mass. Scar formation in tendon isografts or allografts was minimum although 50 to 64 per cent of the grafted tendon collagen turned over in three months time. The xenograft showed an almost complete loss of pre-existing collagen of tendon within a period of one month. The loss of morphological integrity of tendon occurred only in the xenograft. In tendon xenografts there was a marked increase in collagen destruction with almost complete inhibition of collagen synthesis.

The grafting of a known amount of radioactive tendon to non-radioactive animals and its precise removal has permitted the simultaneous quantification of mature collagen destruction, new collagen replacement and change in collagen mass. Any loss in collagen mass as a result of grafting was due chiefly to insufficient or no replacement of the destroyed collagen with new collagen.