Among several different types of phospholipase A₂ (PLA₂), cytosolic PLA₂ (cPLA₂)α and group IIA (IIA) secretory PLA₂ (sPLA₂) have been studied intensively. To determine the discrete roles of cPLA₂α in platelets, we generated two sets of genetically engineered mice (cPLA₂α^−/−/sPLA₂-IIA^−/− and cPLA₂α^−/−/sPLA₂-IIA^+/+) and compared their platelet function with their respective wild-type C57BL/6J mice (cPLA₂α^+/+/sPLA₂-IIA^−/−) and C3H/HeN (cPLA₂α^+/+/sPLA₂-IIA^+/+). We found that cPLA₂α is needed for the production of the vast majority of thromboxane (TX)A₂ with collagen stimulation of platelets. In cPLA₂α-deficient mice, however, platelet aggregation in vitro is only fractionally decreased because small amounts of TX produced by redundant phospholipase enzymes sufficiently preserve aggregation. In comparison, adenosine triphosphate activation of platelets appears wholly independent of cPLA₂α and sPLA₂-IIA for aggregation or the production of TX, indicating that these phospholipases are specifically linked to collagen receptors. However, the lack of high levels of TX limiting vasoconstriction explains the in vivo effects seen: increased bleeding times and protection from thromboembolism. Thus, cPLA₂α plays a discrete role in the collagen-stimulated production of TX and its inhibition has a therapeutic potential against thromboembolism, with potentially limited bleeding expected.