Myelofibrosis with myeloid metaplasia (MMM) is a chronic myeloproliferative disorder due to clonal expansion of a pluripotent hematopoietic progenitor cell with secondary marrow fibrosis. No definitive treatment has as yet been devised for this condition, which shows a marked variability in clinical course. To evaluate whether excessive hematopoietic progenitor cell proliferation could be controlled by recombinant human interferon alpha (rIFN-alpha) and gamma (rIFN-gamma), we studied the effects of these agents on the in vitro growth of pluripotent and lineage-restricted circulating hematopoietic progenitor cells in 18 patients with MMM. A significant increase in the growth (mean +/- 1 SEM) per milliliter of peripheral blood of CFU-GEMM (594 +/- 253), CFU-Mk (1,033 +/- 410), BFU-E (4,799 +/- 2,020) and CFU-GM (5,438 +/- 2,505) was found in patients as compared with normal controls. Both rIFN-alpha and rIFN-gamma (10 to 10(4) U/mL) produced a significant dose-dependent suppression of CFU-GEMM, CFU-Mk, BFU-E, and CFU-GM growth. Concentrations of rIFN-alpha and rIFN-gamma causing 50% inhibition of colony formation were 37 and 163 U/mL for CFU-GEMM, 16 and 69 U/mL for CFU-Mk, 53 and 146 U/mL for BFU-E, and 36 and 187 U/mL for CFU-GM, respectively. A marked synergistic effect was found between rIFN-alpha and rIFN-gamma: combination of the two agents produced inhibitory effects greater than or equivalent to those of 10- to 100-fold higher concentrations of single agents. These studies (a) confirm that circulating hematopoietic progenitors are markedly increased in MMM, (b) indicate that these presumably abnormal progenitors are normally responsive to rIFNs in vitro, and (c) show that IFNs act in a synergistic manner when used in combination. Because rIFN-gamma can downregulate collagen synthesis in vivo, this lymphokine could be particularly useful in the treatment of patients with MMM.