Plasma prostacyclin (PGI2) degradation rates were measured at 1, 5, 15 and 30 min in a group of patients with platelet quantitative disorders of various pathogeneses, including 13 with thrombocytosis, 16 with thrombocytopenia from impaired production in the bone marrow, 11 with thrombocytopenia from peripheral destruction, and 28 normal, healthy persons. Patients with thrombocytosis had a low PGI2 degradation rate, whereas patients with thrombocytopenia due to impaired production had a high PGI2 degradation rate. Of the patients with thrombocytopenia caused by peripheral destruction, six with idiopathic thrombocytopenia purpura (ITP) had a slow PGI2 degradation in contrast to five with systemic lupus erythematosus (SLE) - four concurrently had cryoglobulinemia - who had a rapid PGI2 degradation. The findings suggest that: (1) a platelet-derived substance in the human plasma may have a PGI2 stabilising activity; (2) presence of cryoglobulin or immune complex in plasma may interfere with PGI2 stability.