A drug used to treat B-cell chronic lymphocytic leukemia (CLL) that has not responded to treatment with other anticancer drugs or that has gotten worse. It is also being studied in the treatment of other types of cancer. Fludarabine phosphate blocks cells from making DNA and may kill cancer cells. It is a type of purine antagonist and a type of ribonucleotide reductase inhibitor. Also called Fludara.