Twenty-five pairs of male rabbits of the same variety and strain were joined together in aural parabiosis for 13 to 15 days and then separated from one another. Thirteen pairs of the separated parabiotic twins were surgically united for a second period of parabiosis, less successful than the first due to deficient healing and spontaneous separation of tissue in 5 to 15 days at the site of anastomosis. Within 6 to 90 days after spontaneous or surgical separation of all parabionts, the postparabiotic twins which had been united to one another were cross-grafted with musculofascial transplants. 2 weeks later, the animals were sacrificed and microscopic studies made of the organs and sites of transplantation. These studies disclosed that either one or two periods of parabiosis resulted in the same persistent modification of classical host-homograft interactions. This modification was characterized by the replacement of the customary basic or sensitized homologous incompatibility reactions by a reaction more easily interpreted as one of host-graft tolerance or indifference. The intense inflammatory aspects of the usual host-homograft interactions were absent. Penetration of the musculofascial graft by vascularized mesenchyme derived from the host was retarded. When it occurred, the vascularizing reaction was abnormal and unaccompanied by the customary proliferative, resorptive, and repairative activities of admixed cells of the host and the graft.
These observations provide another means which may be useful in seeking conditions for increasing the probability of successful homografting between members of a genetically impure species.