LMB-2 to Treat Hairy Cell Leukemia

Background: * About 80% of patients with hairy cell leukemia (HCL) have tumor cells that have a protein on their surface called cluster of differentiation 25 (CD25). * The experimental drug LMB-2 is a recombinant immunotoxin that has been shown to kill leukemia and lymphoma cells with the CD25 protein. (A recombinant immunotoxin is a genetically engineered drug that has two parts - a protein that binds or targets a cancer cell, and a toxin that kills the cancer cell to which it binds.) Objectives: * To evaluate the safety and effectiveness of LMB-2 in patients with HCL whose cancer cells contain the CD25 protein. * To evaluate the effects of LMB-2 on the immune system, determine how the drug is metabolized by the body and examine its side effects. Eligibility: -Adults with hairy cell leukemia whose tumor cells have CD25 on their surface Design: * Up to 27 patients may be included in the study. * Patients receive an infusion of LMB-2 through a vein every other day for three doses (days 1, 3, 5), constituting one treatment cycle. * Patients may receive up to six treatment cycles every 4 weeks unless their cancer worsens or they develop unacceptable side effects. * Blood is drawn weekly for various tests. * Before each cycle and in follow-up visits, disease status is evaluated with a physical examination, blood tests, chest x-ray and electrocardiogram. * Before the first cycle, patients may have a computed tomography (CT) scan, echocardiogram (heart ultrasound test) and bone marrow biopsy. With the patient's permission, these tests may be repeated before other cycles also.