Abstract Platelet filamin A (FlnA), encoded by the FLNA gene on the X-chromosome, cross-links actin filaments, links them to the GPIbα subunit of the von Willebrand factor (vWf) receptor in the plasma membrane, and serves as a scaffold for signaling proteins. Mice lacking FlnA were generated. FlnA deficiency is embryonic lethal in hemizygous males due to pericardial and visceral hemorrhage. Heterozygous females are born, but experience premature mortality. The distribution of FlnA-expressing hematopoietic cells in FlnA+/− females was investigated by intracellular flow cytometry. About half of all hematopoietic cells investigated express FlnA, including bone marrow megakaryocytes, suggesting a random X-chromosome inactivation in these cells. However, about 90% of all blood platelets express FlnA. Mice lacking platelet FlnA were further generated by breeding FlnA loxP females with GATA-1 Cre males. FlnA loxP/+ GATA-1 Cre female offspring outnumber FlnA loxP/y GATA-1 Cre males by a ratio of 7:1. FlnA loxP/y GATA-1 Cre males have about 25% of normal blood platelet counts and their platelets are large, lack a normal actin cytoskeleton, and have decreased GPIbα expression, which has an altered distribution by electron microscopy. FlnA-null platelets are cleared rapidly from the circulation when transfused into GATA-1 Cre recipients and have severely impaired functional and signaling responses downstream of the collagen receptor GPVI, i.e. α-granule secretion, integrin αIibβ3 activation and protein tyrosine phosphorylation. Surprisingly, thrombin-mediated GPIbα down-regulation and botrocetin-mediated vWf binding are minimally affected in FlnA-null platelets. The data show the critical nature of FlnA for platelet survival, morphology and function.