A model has been developed for the specific induction of skin and renal allograft prolongation utilizing both antilymphocyte serum and donor bone marrow cells. These studies are directed toward the isolation and characterization of a specific bone marrow cell population which is active in graft prolongation. Cell purification methods involve sedimentation of cells at unit gravity in a 2-4% linear Ficoll gradient with further cell separation by their nylon wool adherence and electrophoretic properties. Once a homogeneous and active cell population is isolated, other methods such as cell receptor analysis, nitrogen stimulation index and stem cell assays will be employed for its characterization. Finally, since marrow is injected a week or more after transplantation in this system, it will be tested after storage at -180 degrees C. These studies could have a significant impact on organ transplantation since they could show that relatively few cells are necessary for the induction of graft prolongation and could lead to other studies related to the mechanism of graft prolongation.