Inclusion Criteria:

- Must have required at least 4 transfusions in the past 12 months

- PNH type III red blood cell (RBC) clone by flow cytometry of >10%

- Lactate dehydrogenase (LDH) level > 1.5 x upper limit of normal

- Platelet count > 100,000/mm3

- Patient taking erythropoietin must be on a stable dose for at least 26 weeks

- Patient taking immunosuppressants must be on a stable dose for at least 26 weeks

- Patient taking corticosteroids must be on a stable dose for at least 4 weeks

- Patient taking coumadin must be at a stable INR for at least 4 weeks

- Patient taking iron supplements or folic acid must be on a stable dose for 4 weeks

- Willing and able to give written informed consent

- Must avoid conception

Exclusion Criteria:

- Mean hemoglobin level prior to transfusion over the previous 12 months is >10.5 gm/dl

- Absolute neutrophil count <500/ul

- Active bacterial infection

- Hereditary complement deficiency

- Participation in any other investigational drug trial or exposure to other
investigational agent, device, or procedures within 30 days

- Pregnant, breast-feeding, or intending to conceive

- History of meningococcal disease

- History of bone marrow transplantation