Human immunodeficiency virus 1-related idiopathic thrombocytopenic purpura (HIV-1-ITP) patients have a 4-fold increased percentage of CD5+ B cells and a 4.8-fold increased percentage of serum immune complexes precipitated by polyethylene glycol (PEG-ICs) compared to control subjects, as reported previously. Since CD5+ B cells produce predominantly IgM rheumatoid factor (RF) vs. Fc of IgG and PEG-ICs contain high levels of IgM, we looked for the presence of RF in the immune complexes of HIV-1-ITP patients. PEG-ICs were adsorbed to protein A and dissociated with acid, and IgM and IgG were purified by gel filtration and affinity chromatography. Solid-phase ELISA was used to measure antibody specificity vs. platelets, Fc, and HIV-1 gp120, p24, and CD4. Dissociated IgG antibody reacted with platelets, HIV-1 gp120, p24, and CD4, but not with Fc. Serum IgG did not react with platelets or Fc but did react with HIV-1 gp120, p24, and CD4. Both PEG-IC IgM and serum IgM reacted with Fc as well as the other four antigens. Control IgM and IgG were unreactive. Isolated IgM from PEG-ICs relocated approximately 50% of the IgG preincubated with IgM to the Vo region of a G200 gel-filtration column. Anti-platelet IgG but not IgM could be affinity-purified from fixed platelets. Both F(ab')2 fragments of anti-platelet IgG and the total PEG-IC bound to platelets in a saturation-dependent manner. F(ab')2 of anti-platelet IgG inhibited 50% binding of PEG-IC to platelets at an F(ab')2/complex ratio of 3:1 (wt/wt). Scatchard analysis revealed two classes of binding sites: high-affinity Kd values of 0.8-1.8 nM and lower-affinity Kd values of 6.6-12.3 nM with respective numbers of binding sites of 44,000-57,000 and 122,000-256,000 (n = 4). Anti-platelet IgG of 6/6 patients precipitated GPIIIa from platelet lysates of surface 125I-labeled platelets. Platelet count correlated inversely with anti-platelet IgG (r = -0.73; P < 0.01; n = 27). Thus, PEG-ICs of HIV-1-ITP patients contain IgM RF, which sequesters serum anti-platelet IgG containing anti-GPIIIa. Anti-platelet IgG contributes to binding of immune complexes to platelets and correlates with thrombocytopenia.