We have used a superparamagnetic microbead selection system to positively select a murine bone marrow CD8+ cell population. The functional ability of these cells to enhance allogeneic bone marrow engraftment was compared with that of fluorescence activated cell sorter purified CD8+ cells. The CD8+ cell population prepared by the microbead selection procedure was as effective as cell sorter purified CD8+ cells in enhancing T cell-depleted allogeneic bone marrow engraftment in lethally irradiated mice. Phenotypic characterization of these cells shows that most of these CD8+ cells express CD3 and the T cell antigen receptor complex.