Studies of the effect of interferon on the growth of colonies of myeloid leukemic blasts, myeloma colony-forming cells and normal hemopoietic precursor cells have shown that interferon does not specifically inhibit the growth of the malignant cells in culture, i.e. the growth of the malignant and the normal precursor cells are inhibited equally. However, interferon markedly reduces the self-renewal capacity of acute myeloid leukemic blasts and myeloma cells. This observation suggested that interferon should be tested for its ability to prolong remissions rather than as a remission-inducing agent. We have tested the ability of interferon alfa-2b (Intron A; Schering-Plough) to prolong remissions induced by busulfan in patients with chronic granulocytic leukemia (CGL). The leukocyte doubling time (Td) and remission duration on no therapy was compared to the values observed during interferon alfa-2b maintenance therapy. Fourteen patients have been started on study and seven have received interferon alfa-2b for three months or more. All seven have shown slowing of the leukocyte Td and prolongation of the remission duration after interferon alfa-2b therapy. A larger study, with longer follow-up, will be required to determine whether interferon alfa-2b therapy will slow or prevent progression of CGL to the blast phase and prolong survival.