A patient with polycythemia vera associated with chromosomal translocation (6;8) (q27;p11) and IgA monoclonal gammopathy was treated with interferon alpha-2b. Interferon alpha-2b induced good hematological control and reduced only IgA levels, but not IgG and IgM levels. In addition, cytogenetic improvement was obtained, the abnormal cells with chromosomal translocation declining from 100% to 50%. Interferon alpha may have selective effects on a malignant clone in polycythemia vera.