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The mechanism of the antineoplastic effect of interferon (IFN) is not known and may result from direct effects on the neoplastic cells themselves, activation of intermediary effector cells, or a combination of these effects. The synthesis of specific proteins is induced in hairy cells when they are exposed to alpha-IFN in vitro. In particular, the called p80, is markedly induced. We investigated this effect in the hairy cells of seven patients in the leukemic phase of hairy cell leukemia who were being treated with subcutaneous (SC) IFN alpha 2b (r-Hu-IFN-alpha 2). Polyacrylamide gel electrophoresis (PAGE) was carried out on [35S]methionine-labeled whole cell lysates visualized by autoradiography and silver staining. Within 2 days of starting IFN therapy, induction of specific protein synthesis, including p80 was seen by [35S]methionine labeling in freshly isolated circulating hairy cells from 6 of 6 patients tested. Therefore, alpha-IFN has a direct biochemical effect on hairy cells in vivo that is similar to in vitro effects, at least with regard to p80 synthesis, although the kinetics of this effect may vary in the two situations.