Recombinant adeno-associated viruses (rAAV) containing only the inverted terminal repeats (ITR) from the wild-type virus are capable of stable integration into the host cell genome, and expression of inserted genes in cultured cells. We have now defined the ability of rAAV to introduce genes into primary hematopoietic progenitors. A vector was constructed containing the coding sequences for beta- galactosidase (beta-gal), including a nuclear localization signal, under the control of a strong viral promotor. Infectious vector particles were prepared by cotransfection of the vector plasmid with a second plasmid that contained the coding sequences for AAV proteins into adenovirus-infected human embryonic kidney cells. These vector preparations transferred and expressed the beta-gal gene in human K562 erythroleukemia and Detroit 6 cells. Positive immunoselection yielded a population of enriched CD34+ cells that were transduced with the rAAV beta-gal vector. Nuclear localized enzyme expression was documented in 60% to 70% of infected cells. Progenitor-derived colonies that developed after 2 weeks in clonogenic cultures were shown to have viral- associated DNA at an estimated copy number of 1 to 2 per cell using a semiquantitative polymerase chain reaction (PCR) method. Integration of AAV into hematopoietic progenitors was documented using wild-type virus, as its genome may integrate at a preferred site on chromosome 19. Our data suggest that rAAV will transfer and express genes in primitive hematopoietic progenitors with high frequency, and support the development of this vector system for therapeutic gene transfer.