Spuriously low electronic platelet counts were obtained on blood collected in EDTA from six patients. Platelet numbers on blood smears and electronic counts performed on blood collected in other anticoagulants were normal. Plasma or sera from these patients agglutinated homologous, as well as autologous platelets, in the presence of EDTA or DTPA, but agglutination did not occur in blood or platelet-rich plasma anticoagulated with heparin, oxalate, or citrate. Agglutination in the presence of EDTA was not inhibited by heparin, α-tocopherol, or N-ethylmaleimide, but was inhibitied by excess EDTA or divalent cation. The agglutinin was temperature independent in five of the six patients and had the physical and immunologic characteristics of a gamma globulin. The clinical significance and mechanism of EDTA-dependent agglutination of platelets is unknown.