Delineation of apoptotic pathways triggered by rATG. Myeloma cell lines and primary myeloma cells were incubated with 250 μg/mL unimmunized rabbit IgG (filled areas) or rATG (open areas) and assayed at 18 hours. Representative histograms are shown (from n = 5 experiments/cell type). Loss of mitochondrial-membrane potential was a prominent feature of all cell lines with the exception of U266, which exhibited primarily caspase-3 activation. Caspase-3 activation was seen to varying degrees in all cell lines and primary cells. All myeloma cell lines and primary cells exhibited subdiploid DNA fragmentation.