Patients must have histopathological evidence of CD25+ CLL or prolymphocytic leukemia (PLL) confirmed by the NIH pathology department. This requires that at least 50% of the peripheral malignant lymphocytes be CD25 positive by fluorescence activated cell sorting (FACS) with anti-CD25 antibody. Positive expression in a FACS assay is defined as more than 2 times the mean fluorescence intensity (MFI) of the control antibody by FACS, or greater than 400 CD25 sites/cell by FACS or radiolabeled binding assay.