The susceptibility of highly purified human CD34+ cells to monocytotropic (Ba-L) and lymphotropic (A018-post) strains of human immunodeficiency virus-1 (HIV-1) was examined. Liquid cultures initiated with fresh immunomagnetically purified CD34+ cells using the K6.1 CD34 monoclonal antibody (MoAb) (K6.1/CD34+) were positive for HIV expression 2 weeks after exposure to HIV-1 Ba-L. These cells were initially greater than 90% CD34+ and had undetectable monocyte contamination by flow-cytometric staining and side-scatter analyses, respectively, and undetectable T-cell contamination by CD3 polymerase chain reaction (PCR) analysis. However, secondary CD34+ liquid cultures reselected from the primary liquid cultures 24 hours after HIV exposure by panning with the ICH3 CD34 MoAb (ICH3/CD34+) and maintained for an additional 14 days were negative for HIV expression. The ICH3-unbound cells were positive for both spliced and unspliced HIV RNA when exposed to HIV-1 Ba-L, and were DNA PCR positive when exposed to either monocytotropic or lymphotropic HIV-1. To further test that CD34+ cells were not infectible by HIV-1, we exposed K6.1/CD34+ cells continuously to HIV-1 in a culture system capable of maintaining and expanding primitive CD34+ cells. HIV-exposed K6.1/CD34+ cells proliferated and expanded as efficiently as uninfected cultures. However, when reselected magnetically using the K6.1 CD34 MoAb after expansion for 7 days, bound K6.1/CD34+ cells were again negative for HIV-1 expression, whereas unbound cells were positive for HIV-1 expression. These findings suggest that a sequential CD34+ cell-selection process, in which the two selections are separated by a brief culture period, can yield a population of CD34+ cells that are not infected with HIV-1. This process may be useful in the design of stem or progenitor cell- based transplantation therapies for HIV infection.