The haemopoietic capacity and graft versus host activity of fractions obtained after separating mouse spleen cell suspensions by centrifugation on a discontinuous albumin gradient were determined and compared with unfractionated spleen cells. One of the fractions obtained showed a 10-fold increase in the concentration of colony forming units and more than a 10-fold decrease of graft versus host activity. Cells from this fraction were found to protect allogeneic irradiated recipients without causing acute or delayed secondary disease. The method seems promising for application to primate bone marrow prior to transplantation, as a means to reduce the severity of acute secondary disease.