RATIONALE: Drugs used in chemotherapy, such as fludarabine, cyclophosphamide, and mitoxantrone, work in different ways to stop the growth of cancer cells, either by killing the cells or by stopping them from dividing. Giving more than one drug (combination chemotherapy) may kill more cancer cells. Monoclonal antibodies, such as alemtuzumab, can block cancer growth in different ways. Some block the ability of cancer cells to grow and spread. Others can find cancer cells and help kill them or carry cancer-killing substances to them. Combination chemotherapy followed by alemtuzumab may be effective in treating chronic lymphocytic leukemia and prolymphocytic leukemia.