Human EBV⁺ B-cell tumors from hu-PBL-SCID mice and from patients with AIDS-PCNSL are susceptible to Fas-induced cell death. Tumor cells were plated in the presence of 10 μg/mL of immobilized M3 anti-Fas MoAb with or without a fivefold excess of soluble M3 MoAb. Cells were then allowed to incubate for 24 hours and procured. Wells were scored for the number of cells/well that excluded vital dye. Three human EBV⁺ B-cell tumors of hu-PBL-SCID mice and two human EBV⁺ B-cell tumors from patients with AIDS-PCNSL were analyzed. Cells were enumerated from two wells under each experimental condition for each tumor. Values represent the mean percent ± SEM of viable cells/well.