Mice pretreated with injections of allogeneic platelets were found to mount a decreased antibody response upon challenge by lymphocytes of the same donor strain. This decrease was mediated by platelets themselves, and not by leucocytes and red cells contaminating the platelet suspension. It affected specifically antibodies reactive with H-2 class I antigens present on donor platelets. This phenomenon may be related to the lack of class II or some non-H-2 antigens on platelets, and/or to properties of their class I antigens (soluble molecules adsorbed from the plasma). These findings emphasize the potential usefulness of purified platelet transfusions preceding organ transplantation in man.