Detection of apoptosis in CLL cells (patient UPN28a) as detected by annexin-V/PI assay after continuous exposure to medium ([A] 4 hours and [C] 24 hours) or 0.18 μmol/L flavopiridol ([B] 4 hours and [D] 24 hours). Phosphatidyl serine is restricted to the internal cell membrane but is externalized early during the process of apoptosis. Cellular uptake of propidium iodine indicates a disrupted cellular membrane generally observed during late apoptosis or with cell necrosis. Increased annexin binding to exposed phosphatidyl serine in quadrant 3 of (B) and (D) as compared with (A) and (C) is supportive of flavopiridol inducing cytotoxicity at least in part through the pathway of apoptosis in human CLL cells.