Peripheral-blood mononuclear cells from patients with PNH have strong cytolytic activity. Peripheral-blood mononuclear cells were isolated from either patients with PNH (A) or healthy donors (B) and their cytolytic activity was analyzed in a 4-hour redirected killing assay using the FcγR⁺ murine cell line P815 in the absence (none) or in the presence of mAbs recognizing the indicated surface molecules. Anti-CD54 mAb was used as an isotype-matched control mAb. The PNH donor samples (PNH6 and PNH7) used in this experiment had a low number of CD16⁺ NK cells (9 cells/μL in PNH6 and 24 cells/μL in PNH7) but sizeable proportions of CD3⁺IRS⁺ cells (109 cells/μL in PNH6 and 112 cells/μL in PNH7). Cytolytic activity of peripheral-blood lymphocytes from 2 representative healthy donors (H1 and H2) is shown for comparison (B). Results are expressed as % ⁵¹ Cr-specific release at an E/T ratio of 10:1 mean of triplicate samples; bars indicate standard deviation of triplicate samples. A comparable difference between PBMCs of patients with PNH and healthy donors was found also at 20:1 and at 40:1 E/T ratios.