Retroviral-mediated gene transfer of human beta-globin provides a model system for the development of somatic gene therapy for hemoglobinopathies. Previous work has shown that mice receiving a transplant of bone marrow cells infected with a retroviral vector containing the human beta-globin gene can express human beta-globin specifically in erythroid cells; however, the level of expression of the transduced globin gene was low (1% to 2% per gene copy as compared with that of the endogenous mouse beta-globin gene). We report here the construction of a recombinant retrovirus vector encoding a human beta- globin gene fused to the 4 major regulatory elements of the human beta- globin locus control region (LCR). The LCR cassette increases the level of expression of the globin gene in murine erythroleukemia cells by 10- fold. To study the level of expression in vivo, mouse bone marrow cells were infected with virus-producing cells and the transduced cells were injected into lethally irradiated recipients. In the majority of provirus-containing mice (up to 75%), expression of human beta-globin in peripheral blood was detected at least 3 to 6 months after transplantation. Twelve animals representative of the level of expression of the transduced gene in blood (0.04% to 3.2% of the endogenous mouse beta-globin RNA) were selected for further analysis. A range of 0.4% to 12% of circulating erythrocytes stained positive for human beta-globin protein. Based on these values, the level of expression of the transduced gene per cell was estimated to be 10% to 39% of the endogenous mouse beta-globin gene. These data demonstrate that fusion of the LCR to the beta-globin gene in a retroviral vector increases the level of beta-globin expression in murine erythroleukemia cells and suggest that high-level expression can be obtained in erythroid cells in vivo after transduction into hematopoietic stem cells.