Our previous finding that normal serum immunoglobulins bind to internal platelet proteins on Western blots led us to further identify these proteins and determine the possible significance of autoantibodies against them. A 95-Kd protein reactive with immunoglobulins in most normal sera and easily confused with gpIIIa was shown to be a fragment of vinculin generated by calpain proteolysis. Identity was established by peptide sequencing of the protein purified from platelets stored without specific protease inhibitors. Normal immunoglobulins bound intact vinculin (117 Kd) and metavinculin (152 Kd), and their 105-, 95- , and 80- to 85-Kd proteolytic fragments. IgG in 89%, and IgA and IgM in 100% of normal sera reacted in titers of 10 to 1,000 with purified vinculin. In addition, IgG in 79%, and IgA and IgM in 93% of normal sera reacted in titers of 10 to 5,000 with talin (235 Kd), another cytoskeletal protein, and its 200-Kd proteolytic fragment. IgGs in sera from animals of several different phylogenetic classes also reacted with human vinculin and talin on Western blots. Frequency of occurrence, titers, and classes of antivinculin and antitalin autoantibodies in patients with thrombocytopenia did not differ discernibly from those of normal individuals. These antibodies had no effect on platelet aggregation or clot retraction, and no apparent pathogenic significance, but their widespread presence and the variability in extent of proteolysis of platelet preparations used for Western blots can complicate interpretation of patterns obtained with sera from patients with presumed immune-mediated thrombocytopenias.