We prepared a heterohybrid cell line that secretes a human IgM monoclonal autoantibody that recognizes an antigen found on thrombin-activated or stored platelets. The surface expression of the epitope recognized by this autoantibody, 5E5, increases with time as platelets age in vitro, suggesting that it may represent a senescence or activation-specific antigen. 5E5 binds to the purified platelet membrane glycoprotein (GP) IIb-IIIa complex in an enzyme-linked immunosorbent assay (ELISA). In an immunoblot technique, 5E5 binds to a protein with an apparent mol wt of 95,000, which is identical to that of GPIIIa under nonreduced conditions. In crossed immunoelectrophoresis (CIE), the predominant antigen recognized by 5E5 is contained in the GPIIb-IIIa precipitin arc. An additional precipitin arc recognized by 5E5 is often observed only on gels derived from lysates of platelets stored under blood bank conditions for greater than 3 days. These findings illustrate the usefulness of human monoclonal antibodies for the identification of membrane neoantigens expressed as a result of platelet activation or revealed as platelets age in vitro.