A retroviral expression vector (N2) containing the selectable gene, neoR, has been used to determine the optimal conditions for infecting murine hematopoietic progenitor cells at high efficiency. After infected bone marrow cells were introduced into lethally irradiated mice, the presence, stability, and expression of the vector DNA sequences were analyzed either in individual spleen foci 10 days later or in the blood, bone marrow, and spleens of mice 4 months later. When bone marrow cells were cultured in medium containing virus with titers of more than 10(6) colony-forming units per milliliter in the presence of purified murine interleukin-3, more than 85 percent of the resulting foci contained vector DNA. This proviral vector DNA was intact. Efficient expression of the neoR gene was demonstrated in most of the DNA-positive foci examined. The spleens of reconstituted animals (over a long term) contained intact "vector DNA" and the blood and bone marrow expressed the neoR gene in some animals. Thus, a retroviral vector can be used to introduce intact exogenous DNA sequences into hematopoietic stem cells with high efficiency and with substantial expression.