Paroxysmal nocturnal hemoglobinuria (PNH), an acquired clonal disorder is manifested by failure of hematopoietic cells to express phosphatidylinositol glycan-anchored proteins (PIG-AP). Since the PIG-A mutation is present at the stem cell level, all cell lines may be affected. Although the pathogenesis of hemolytic anemia in PNH is related to the absence of CD55 and CD59 molecules on the membrane of red cells, the mechanism responsible for the increased incidence of thrombotic events in PNH is not clear. In this study we measured two glycosylphosphatidylinositol (GPI)-linked molecules on platelets (CD55 and CD59) and two GPI-linked proteins on neutrophils (CD14 and CD16), comparing their expression on normal and PNH patients. Using two-color flow cytometric analysis with antibodies directed against CD42b and CD41a, we found that CD55 and CD59 were constitutively expressed by normal fresh platelets, but that the expression levels decreased during the five day storage of platelets. A substantial population of platelets lacking the GPI-linked proteins were detected in most cases. We demonstrated varying degrees of deficiency in the expression of GPI-anchored molecules with neutrophils, monocytes and platelets with the highest proportion of deficient cells found within monocytic lineage. Similar numbers of platelets with the PNH phenotype and normal platelets expressed activation markers before and after exposure to platelet agonists. Flow cytometry is more sensitive than Ham's test in monitoring expression of PNH in platelets. Differences in the numbers of circulating GPI-deficient platelets and myeloid cells suggest that either the survival of platelets and mature myeloid cells differs or megakaryocytopoeisis is abnormal within the GPI-deficient clone.