We have long known that immune complexes acti vate platelets by interacting with the platelet Fc receptor for immunoglobulin G (IgG)(FcγR), and it is becoming increasingly obvious that antiplatet antibodies may exert their biologic effects in a like manner. Thus, as part of a monograph on platelet antibodies, we shall review what is known about the platelet FCγR and its potential role in antibody-mediated platelet disorders. We shall focus on the details of the 40 kDa protein referred to as FcγRII, excluding from our discussion a description of other mol ecules that have been suggested as candidate FcγR but for which considerably fewer data exist. Details of these other molecules were summarized in the last review of the platelet FcγR 6 years ago. 1 Also, we exclude from our discussion the platelet low affinity Fc receptor for IgE (FcεRII, CD23)(for review, Delespesse et al2).