Lymphoma B cells are resistant to CD95-mediated apoptosis in an assay detecting early stages of apoptosis. Flow cytometric analysis of Jurkat cells (A), nontumoral B cells (B; Donor NT-1), and lymphoma B cells (C through E). Lymphoma B cells were obtained from MCL-4 (C), FL-3 (D), DLCL-3 (E). Cells were incubated with 1 μg/mL anti-CD95 MoAb CH-11 or with IgM control MoAb for 4 hours (A) or 18 hours (B through E) and then double-stained wirth annexin V-FITC/PI and analyzed by flow cytometry. Numbers within dot plots represent the percentages of cells in early apoptosis (lower right, annexin V⁺/PI-) and in late apoptosis or in necrosis (upper right, annexin V±/PI⁺). The specific apoptosis, that is the percentage of viable cells undergoing apoptosis with the inducing agent, was calculated according to the following formula: ((D_CH-11 − D_IgM)/100 − D_IgM) ×100, where D represents the percentage of cells dying during the culture by apoptosis or necrosis.