Abstract Platelet-associated immunoglobulin was measured by the use of fluorescent anti-IgG antibody. The method is simple, rapid, and sensitive and provides a precise quantitive assay of bound (direct) and free (indirect) IgG with platelet specificity. We have evaluated this test in 30 normal volunteers and in 50 patients with immune and nonimmune, treated and untreated thrombocytopenias. All patients with immune thrombocytopenias (acute and chronic idiopathic thrombocytopenic purpura and systemic lupus erythematosus) having platelet counts <100,000/μl had elevated levels of platetet-bound IgG, and 86% had also positive results in the indirect assay. All patients with nonimmunological thrombocytopenias showed normal results in the direct and indirect assay of platelet-associated immunoglobulin. In patients studied repeatedly during the course of their illness, an inverse relation was found between platelet count and level of platelet-bound IgG. Patients with systemic lupus erythematosus presented clear exceptions to this rule. Investigations of the absorbability of platelet autoantibodies and alloantibodies showed that this assay can readily differentiate between these two antibody species and can also identify specificities of alloantibodies.