A method of enhancing tolerance of a porcine transplant in a xenogeneic recipient by administering porcine bone marrow cells to the recipient and of enhancing proliferation and engraftment of the porcine bone marrow cells by exposing said cells to at least one substantially pure porcine cytokine and porcine cytokines that are substantially free of other porcine proteins and preferentially enhance the proliferation and engraftment of porcine bone marrow cells in the presence of bone marrow cells of other species. Protein and DNA sequence(s) for such porcine cytokines.