Vector-containing medium harvested from murine packaging cell lines has been shown to contain factors that can negatively influence the transduction and maintenance of hematopoietic stem cells. Thus, we generated a human packaging cell line with a gibbon ape leukemia virus pseudotype (Phoenix-GALV), and we evaluated vectors produced by Phoenix-GALV for their ability to transduce hematopoietic progenitor/stem cells. In 3 baboons, we used a competitive repopulation assay to directly compare GALV-pseudotype retrovirus vectors produced by either Phoenix-GALV or by the NIH 3T3–derived packaging cell line, PG13. In 3 additional baboons we compared Phoenix-GALV–derived vectors to more recently developed lentiviral vectors. Gene transfer efficiency into hematopoietic repopulating cells was assessed by evaluating the number of genetically modified peripheral blood and marrow cells using flow cytometry and real-time polymerase chain reaction. Transduction efficiency of hematopoietic repopulating cells was significantly higher using the Phoenix-GALV–derived vector as compared with the PG13-derived vectors or lentiviral vectors, with stable transduction levels up to 25%. We followed 2 animals for more than one year. Flow cytometric analysis of hematopoietic subpopulations in these animals revealed transgene expression in CD13⁺ granulocytes, CD20⁺ B lymphocytes, CD3⁺ T lymphocytes, CD61⁺ platelets, as well as red blood cells, indicating multilineage engraftment of cells transduced by Phoenix-GALV–pseudotype vectors. In addition, transduction of human CD34⁺ cells was significantly more efficient than transduction of baboon CD34⁺ cells, suggesting that Phoenix-GALV–derived oncoretroviral vectors may be even more efficient in human stem cell gene therapy applications.