Abstract: 3,4-Dihydroxyacetophenone (DHAP) was shown to significantly inhibit the release of TXB₂ from rabbit platelet aggregate induced by collagen or AA either in vitro or in vitro in a dose-dependent pattern. The results from bioassay and radioimmunoassay were parallel. The amount of TXB₂ released from rabbit platelets induced by AA was 3～6 times higher than that induced by collagen. However, the level of TXB₂ spontaneously released from rabbit platelets was very low. In addition, the effect of DHAP on inhibition of TXB₂ release from platelets induced by AA was much stronger than that by collagen. Whether DHAP is a cyclooxygenase inhibitor or thromboxane synthetase inhibitor needs further study. Direct determinations of cycloxygenase and thromboxane synthetase activity are to be carried out before a definite conclusion can be made.