To test whether urate crystal‐membrane protein interactions mediate platelet stimulation, platelet membrane proteins were radiolabeled by lactoperoxidase, extracted with 1% Triton X–100, and incubated with urate crystals. The crystal‐associated and supernate fractions were isolated and analyzed by sodium dodecyl sulfate–polyacrylamide gel electrophoresis (SDS–PAGE) and by 2‐dimensional isoelectric focusing followed by SDS‐PAGE. Four of the lactoperoxidaseradiolabeled polypeptides that associated with urate crystals had reduced molecular weights and pIs of M_r = 105,000, pI 4.9; M_r = 123,000, pI = 4.9; M_r = 123,000, pI = 5.3; and M_r = 132,000, pI = 4.8–6.3, respectively. These proteins were characterized with regard to their labeling intensities, apparent isoelectric points, apparent molecular weights (reduced and nonreduced), lectin‐binding properties, carbohydrate‐ and protein‐staining characteristics, and presence or absence in Glanzmann's thrombasthenia. They have been identified as derived from glycoproteins Ib, IIb, and III (Phillips‐Agin nomenclature) and an unidentified membrane protein. To test whether removal of these proteins would result in a diminution of platelet response to urate, intact platelets were digested with chymotrypsin, resulting in cleavage of more than 80% of these proteins and a 5‐fold reduction in secretory responsiveness to urate crystals. Response to a second platelet stimulus, collagen, was unaffected, indicating an intact secretory mechanism. In addition, when platelets were preincubated with F(ab′)₂ fragments of an antibody directed against these proteins, platelet secretory response to urate was inhibited by 50%, whereas the responses to collagen and thrombin were unaffected. Thus, membrane proteins appear to mediate platelet stimulation by urate crystals.