In this study we describe a new monoclonal antibody (MoAb PL.1) against rat platelets. Immunohistology of various rat tissues showed staining of platelets, especially in the spleen, and staining of megakaryocytes in bone marrow and spleen red pulp. In the liver small platelet aggregates and endothelial cells were stained. After in-vivo administration of MoAb PL.1 an acute severe thrombocytopenia was observed. In general the distribution of the antibody and/or antibody-coated platelet aggregates showed the same pattern as after in-vitro incubation, i.e. staining of rat platelets and platelet aggregates in spleen red pulp, and staining of megakaryocytes in spleen and bone marrow. Platelet aggregates were observed in the liver and electron microscopy indicated that they were associated with Kupffer cells. Furthermore, liver endothelial cells were positively stained. Comparison of the molecular weight of the antigens recognized by this MoAb and by human anti-platelet MoAbs, as well as comparison of staining patterns of megakaryocytes indicated that MoAb PL.1 is probably directed to a GPIIb/IIIa complex analogue. Since MoAb PL.1 is of the non-complement-binding mouse IgG1 isotype, it can be used for studying clearance of platelet aggregates by Fc-receptors of the MPS. It also promises to be a useful tool in the study of platelet involvement in rats with experimental nephritis.