A "cold" platelet agglutinin is described which is present in plasma and serum, is greater than 100,000 in molecular weight, appears to be a protein of the immunoglobulin type, is adsorbed by platelets in the absence of bivalent cations, and reacts only at temperatures below 34°C. It agglutinates homologous and autologous platelets but appears to have no effect on in vivo platelet function. It is seen in patients with systemic disease and can cause spuriously low platelet counts.