Measurement of EBV-specific CTL response by ELIspot following myeloablative and nonmyeloablative transplantation. Panel A shows patients' responses following myeloablative transplantation and panel B shows patients' responses following nonmyeloablative transplantation. The proportion of PBLs (with 95% CI) responding to specific peptide stimulation as determined in ELIspot assays at different time points after transplantation. Individual cells producing IFN-γ in response to overnight stimulation with synthetic peptide, dimethyl sulfoxide (DMSO) solvent or phytohemagglutinin (PHA) were detected in ELIspot assays (Millipore, Bedford, MA; and Mabtech, Stockholm, Sweden) as dark spots after the cells had been discarded. The spots were counted under a dissection microscope. Frequencies exceeding 10 responders/million PBLs were considered positive. The background IFN-γ release from lymphocytes not stimulated with peptide is subtracted from the peptide-specific responses. The peptide targets are listed in Table 1. Broken bars represent estimates of the magnitude of responses too numerous to count. Y-axes show responders/million PBLs.