It is estimated that 30-50% of patients with CLL have tumors that express cluster of differentiation 25 (CD25) (Tac or IL2R). Normal resting T-cells are not sensitive to LMB-2 due to insufficient CD25 expression. LMB-2 is an anti-CD25 recombinant immunotoxin containing variable domains of MAb anti-Tac and truncated Pseudomonas exotoxin. A phase I trial at National Cancer Institute (NCI) found that the maximum tolerated dose (MTD) of LMB-2 was 40 microg/Kg IV given every other day for 3 doses (every other day (QOD) x3) with prophylactic IV fluid. The most common adverse events were transient fever, hypoalbuminemia and transaminase elevations. In that trial, one of eight patients with chronic lymphocytic leukemia had a partial remission. The other seven CLL patients had stable disease. In addition, four of four patients with hairy cell leukemia had responses (1 complete response (CR), 3 partial response (PRs)) and 3 other patients had PRs (1 cutaneous T-cell lymphoma (CTCL), 1 healthy donor (HD), 1 acute T-cell leukemia/lymphoma (ATL)). Because LMB-2 is cytotoxic to cells expressing CD25, CD25+ CLL patients are good candidates for further testing with LMB-2.