Autologous lymphocytes decrease the proportion of BM trisomy B cells while Fas antagonist increases their growth. Fas agonist increases the proportion of trisomy 8 cells in short-term culture. BMMCs were obtained from 10 patients with trisomy 8, 4 patients with monosomy 7, and 4 healthy controls. Samples were divided into 2 aliquots, one of which was incubated with autologous PBLs for 4 hours, as described in “Patients, materials, and methods.” Samples were subsequently depleted of lymphocytes and placed in semisolid media with growth factors for short-term culture. Colonies were counted, and FISH was performed. The number of trisomy 8 cells and the percentage of trisomy 8 cells were decreased by lymphocyte cocultivation, with little effect seen in diploid cells (A). When one aliquot was depleted of T cells using CD3-specific magnetic beads before short-term culture and compared with the non–T cell–depleted sample, T-cell cultures consistently showed increased trisomy 8 progenitor–derived cell growth (B, left). T-cell depletion had little effect on the growth of cytogenetically normal cells (B, right). When samples of BM were plated in long-term culture with autologous lymphocytes, with and without Fas antagonist (ZB4 mAb), trisomy 8 cells increased compared to karyotypically normal cells (C).