Percentage of apoptotic nuclei among peripheral blood mononuclear cells stimulated for 48 hours with PHA, stained with Hoechst stain, and assessed for nuclear morphology under a fluorescent microscope. Postchemotherapy donor's (PC) cells consistently had a higher level of apoptotic nuclei after stimulation than did normal (N) control cells. Data from the 9- and 15-month time points represent patients in the FLAC/paclitaxel protocol (#9447 and 9306, respectively). The 3-month follow-up assay was done on a patient in a similar chemotherapy protocol (ACT).