Effects of anti-Fas IgM ligation on neutrophil apoptosis. Human neutrophils were incubated with increasing concentrations of anti-Fas IgM, CH-11, and assessed for apoptosis after 18 hours. Apoptosis was measured by flow cytometry using PI DNA staining with representative histograms shown in (A) and the percentage of cells with apoptotic DNA plotted in Fig 1B. (*P < .05 v ng/mL anti-Fas). The high-intensity peak at a relative fluorescence intensity of 500 Ln represents normal DNA, whereas the lower peak between 10 and 100 Ln intensity indicates apoptotic or fragmented DNA with reduced uptake of propidium iodide. Results are the mean ± SD for n = 5 experiments with three separate donors of neutrophils.