Disruption of the actin cytoskeleton markedly augments α-granule secretion stimulated by weak agonists. Platelets were incubated in the presence (□) or absence (▪) of 4 μM latrunculin A for 15 minutes. Platelets were then incubated with the indicated agonists for 10 minutes and subsequently assayed for α-granule secretion and dense granule secretion. α-Granule secretion was monitored by assaying for P-selectin surface expression by flow cytometry. Dense granule secretion was monitored by assaying for ¹⁴C-serotonin release from ¹⁴C-serotonin-loaded platelets. Data are expressed as fold increase compared with samples treated with vehicle alone.