PRIOR CONCURRENT THERAPY: Biologic therapy: Concurrent leukapheresis allowed during the first month of study No concurrent allogeneic bone marrow transplantation Concurrent anagrelide allowed during the first 3 months of study Chemotherapy: See Disease Characteristics Concurrent hydroxyurea allowed only during the first 3 months of study Endocrine therapy: No concurrent systemic steroids for more than 2 weeks Radiotherapy: Not specified Surgery: Greater than 4 weeks since prior major surgery and recovered Other: No other prior investigational agents No other concurrent investigational drugs