Changes in OD as a function of time in microcultures of leukemic cells: control and drug-treated HL-60 cells (A), freshly isolated ANLL cells (B), and ALL cells (C). The OD data were imported from SOFTmaxPro software to a spreadsheet program (MicroCal Origin) and plotted against time. Exponentially growing control cultures of HL-60 cells display a consistent OD increase over time which is due to an increase in cell number in the microculture. OD versus time curve from control ANLL cells shows a pattern of a slow autonomous cell growth while control ALL cells do not grow, resulting in a flat OD versus time curve. In all three cell types, exposure to IDR, MTA, or DNR causes apoptosis which is indicated by steep increases of the OD in cultures. Extent of apoptosis in HL-60, ANLL, and ALL cells varies and is proportional to the slope of the best fit line (shown as dotted lines) of the apoptotic curves. OD in the blank wells was 0.045 ± 0.001.