Employing immunofluorescent staining methods, platelets from several adult patients with chronic idiopathic thrombocytopenic purpura, with systemic lupus erythematosus, and with classical rheumatoid arthritis stained positively with antisera to C3(β1C and C3dg(α2D). In some cases, platelet-bound C3dg antigens, but no evidence of other C3 antigens were observed. Platelets that were positive for C3 and/or C3dg were always positive for IgG or for both IgG and IgM. Platelets from normal subjects stained negatively for C3, C3dg, IgG, and IgM. Furthermore, various incubation experiments were performed to investigate the uptake of C3 and its subfragments on normal platelets. Positive platelet staining for C3 was obtained after incubation with normal and C3b-containing sera, but not after incubation with native C3. Platelets incubated with C3c(β1A)/C3dg-containing serum gave a positive staining for C3dg, but not for C3c. None of the sera stained positively for IgG or IgM. Thus, in contrast to platelets from several of the patients, no in vivo binding of C3 antigens on normal platelets could be established. However, in vitro they could take up both C3b and C3dg, apparently by specific binding to platelet surface structur