Leukemic cell-mediated cytotoxicity. Median percentage apoptosis of Jurkat cells by leukemic samples from 27 patients at E/T ratios of 30:1 (black) and 10:1 (gray). Each sample was tested in duplicate at each time and in at least 2 to 5 separate experiments. Cytotoxicity was assessed by the JAM test, which measured radioactivity released by DNA fragmentation. The target cell Jurkat DNA was labeled with 3H-thymidine, so that the test reflects fragmentation of target cell DNA in response to a signal induced by leukemic cells. The apoptosis induced by PBMC of normal donors, used as a negative control, never exceeded 10% at the highest E/T ratio.