The aim of the study was to assess whether other cells, besides erythrocytes, may influence the cytotoxic effect of mafosfamide (maf) during ex vivo bone marrow purging from residual tumor cells before autologous transplantation. It was shown that the presence of normal granulocytes, blast cells from acute myeloid leukemia-patients (AML) and lymphoma cells from patients with chronic lymphocytic leukemia (CLL) during maf incubation did not change the maf-induced growth inhibition of CFU-GM. Similar observation was made in experiments with resting lymphocytes. However, when phytohaemagglutinin- and pokeweed mitogen-preincubated lymphocytes were present in the marrow cell suspension, significant decline of the maf-related CFU-GM cytotoxicity was observed. These results suggest that besides erythrocytes also the activated lymphocytes in the marrow mononuclear suspension may change the final effect of maf purging.