Elimination of contaminating malignant cells is crucial to avoiding cancer relapse in association with transplantation of autologous spermatogonial stem cells. In the clinical setting, there is presently no effective procedure for separating testicular cells from cancer cells. Here, CD4, a selective surface marker for Roser's rat leukaemic T-cells, was utilized to eliminate cancer cells from testicular cell samples from leukaemic piebald variegated (PVG) rats by magnetic-activated cell sorting (MACS). All animals receiving MACS-selected testicular cells died within 14-15 days. Only one-third of the contaminating leukaemic cells could be removed from testicular samples. An increase in antibody concentration enhanced the proportion of leukaemic cells removed from 27 to 49%. Variations in the cell size and expression of surface antigens on testicular leukaemic cells were the major obstacles to purification based on this marker. It is concluded that MACS does not prevent transmission of leukaemia to syngenic PVG rats when cells from leukaemic testes are used for testicular cell transplantation.