Immortalized human stromal cell lines sustain and expand human hematopoietic precursor cells. The precursor cells are obtained from a blood product and inoculated into a culture medium conditioned by exposure to a human stromal cell line. Preferred human stromal cell lines secrete SCF, LIF, MIP1.alpha., and IL-6, as exemplified by a human stromal cell line designated HS-1. The conditioned culture medium may be supplemented with additional growth factors, such as interleukin-3. After expansion the human hematopoietic precursor cells are harvested and returned to a patient or frozen and stored. The immortalized human stromal cell lines can also be used as feeder layers in ex vivo bone marrow cultures or in colony forming assays.