IgG, IgA, IgM, and albumin are primarily known as plasma proteins. Their presence in platelets is poorly understood. The total platelet content of IgG, IgA, and albumin, measured in solubilized platelets by an enzyme-linked immunosorbent-assay (ELISA) technique, was greater than 90% secreted after stimulation by thrombin, consistent with an alpha-granule location. The platelet concentrations of these proteins correlated with their plasma concentrations in normal subjects and over a wide range of abnormalities in patients with IgG or IgA myeloma or liver cirrhosis. IgM was not detectable in normal platelets but was measurable and related to the plasma IgM concentration in patients with macroglobulinemia. In patients with idiopathic thrombocytopenic purpura (ITP), the platelet concentrations of IgG, IgA, and albumin were all twofold to threefold higher than normal despite normal plasma concentrations. Platelet surface IgG, measured by 125I-monoclonal antibody binding, constituted less than 1% of the total platelet IgG, and it appeared to be a pool distinct from the alpha-granule IgG since its concentration in normal subjects and patients did not correlate with either plasma or total platelet IgG concentrations. These observations are consistent with hypotheses that megakaryocytes incorporate plasma proteins into developing alpha-granules by pinocytosis and that the increased ratio of platelet to plasma of IgG, IgA, and albumin in ITP may reflect a younger average age of these platelets.