The nature of platelet- bindable immunoglobulins (PB-Ig) in serum has been investigated. PB-IgG, -A and -M were measured by an ELISA using platelets coated on microtitre plates. This assay detected alloantibodies at high serum dilutions. In 32 patients with idiopathic thrombocytopenic purpura (ITP) or systemic lupus erythematosus (SLE) raised levels of at least one PB-Ig class were found in 18. To distinguish binding due to immune complexes, the molecular weight of PB-IgG was studied by gel filtration on Sepharose 4B. In sera from patients with ITP and SLE, PB-IgG with Mr of primarily 150 Kd was observed, compatible with monomeric IgG antiplatelet antibodies. Levels of PB-IgG in serum were not related to total serum IgG. In sera from the patients with SLE and some with ITP (most of whom had several of the features of SLE), PB-IgG with Mr of 200 Kd - greater than 1000 Kd was seen. In heat-aggregated preparations of normal IgG, PB-IgG with Mr up to 1000 Kd was also found. Rabbit IgG was able to block PB-IgG in fractions of high molecular weight in purified normal IgG, heat-aggregated normal IgG and in patient serum, but had no effect on the 150 Kd peak. In whole serum from patients who had high molecular weight PB-IgG, the inhibitory effects of rabbit IgG were much less than in isolated high molecular weight column fractions. Thus although the majority of PB-IgG is monomeric antiplatelet antibody, some PB-IgG with higher molecular weight, characteristic of immune complexes, occurs in sera of some patients with autoimmune thrombocytopenia and it makes a small contribution to PB-IgG levels measured in whole serum.