Primary AML samples were incubated with increasing concentrations of SGN-CD33A or GO for 96 hours before flow cytometric analyses of live/dead cells. Data are expressed as IC₅₀, the concentration of ADC required to give a 50% reduction in cell viability. CD33 expression on bulk tumor cells and the CD34⁺ subset were determined by flow cytometry and expressed as MFI. The CD34⁺ subset (data not shown) expressed CD33 if this antigen was found on the bulk tumor cells and expression levels in these 2 groups were comparable. +, fluorescence in the presence of 2 different MDR pump inhibitors more than 2-fold above background; MDR, multidrug resistance; MFI, mean fluorescence intensity.