A new retroviral vector is characterized in which the enhancer in the long terminal repeat of myeloproliferative sarcoma virus (MPSV) is combined with the strong and universally active immediate-early human cytomegalovirus (hCMV) enhancer. We demonstrate that insertion of the hCMV enhancer increases the amount of vector-specific mRNA in various rodent cell lines and in human diploid fibroblasts, by at least 3-5-fold. The vector may be particularly useful if high expression of two genes is desired.