The increase in free cytosolic calcium is concentration dependent. Washed human platelets were stained with 1μM Fluo4/AM in HEPES-buffered saline for 30 minutes. Intracellular calcium levels were continuously monitored by flow cytometry. After establishing a baseline fluorescence signal for 1 minute, ABT-263 was added at 1, 3, or 10μM and the calcium response was monitored for an additional 3 minutes.