Split cell chimerism in patients more than 1 year after HSCT. Of 42 patients, 24 (57%) had 100% donor chimerism in all cell lines: 15 of 26 (58%) in the fludarabine group and 9 of 16 (56%) in the cyclophosphamide group. The rest had stable mixed chimerism. There was no very low level chimerism (< 10%) in the T-cell lineage and very little in the B and myeloid cell lineages. There was significantly better T-cell chimerism in the group receiving fludarabine (P = .038). T indicates T-cell lymphocytes; B, B-cell lymphocytes; M, myeloid cells; Flu, fludarabine; and Cy, cyclophosphamide.