Since serum tryptase levels are elevated in some patients with myeloproliferative disorders, we examined their utility in identifying a subset of patients with hypereosinophilic syndrome (HES) and an underlying myeloproliferative disorder. Elevated serum tryptase levels (> 11.5 ng/ mL) were present in 9 of 15 patients with HES and were associated with other markers of myeloproliferation, including elevated B 12 levels and splenomegaly. Although bone marrow biopsies in these patients showed increased numbers of CD25 ؉ mast cells and atypical spindleshaped mast cells, patients with HES and elevated serum tryptase could be distinguished from patients with systemic mastocytosis and eosinophilia by their clinical manifestations, the absence of mast cell aggregates, the lack of a somatic KIT mutation, and the presence of the recently described fusion of the

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BL22 was well tolerated and highly effective in HCL, even after one cycle. Phase II testing is underway to define the efficacy with one cycle and to study safety when additional cycles are needed for optimal response.