Various chemical compounds have been described to induce photosensitization of tumor cells resulting in cell death. We studied the effect of merocyanine-540 (MC-540) on both leukemic and normal bone marrow (BM) cells. Acute promyelocytic leukemia (HL-60) and common acute lymphoblastic leukemia antigen-positive non-T, non-B acute lymphoblastic leukemia (Reh) cell lines were incubated with MC-540 and simultaneously exposed to white light. Normal human BM and mixtures of leukemic cells with BM cells were treated under similar conditions. At constant illumination rates of 50,000 lx, significant (at least 4 to 5 logs) tumor cell destruction was obtained with MC-540 concentrations of 20 micrograms/ml or more for HL-60, and 10 micrograms/ml or more for Reh cells. Incubation of BM under equivalent conditions preserved 18.0% of granulocyte-macrophage colony-forming units and 14.2% of erythroid burst-forming units. Similar results were obtained when tumor cells were mixed with irradiated BM and then treated with MC-540. In summary, cell photosensitization with MC-540 has a selective cytotoxic effect towards leukemic cells and therefore may be useful for purging tumor cells from autologous BM.