Neutrophil apoptosis. Neutrophil apoptosis was determined as described in Materials and Methods. Apoptosis of isolated neutrophils, after 5 days of G-CSF therapy, was significantly reduced by 9 hours of culture and remained lengthened throughout the assay time (30 hours). Results are expressed as percent apoptosis and represent mean ± SEM, n = 8 (matched pairs); C, control; P, treated volunteer.