Miniature swine are being used as a large animal model in which cultured and retrovirus-transduced hematopoietic stem cells (HSC) can be tested in a reproducible manner for their long-term in vivo repopulating ability. As part of these studies, long-term bone marrow culture (LTBMC) and progenitor colony assay systems were developed and used to characterize the in vitro growth potential and in vivo frequency of hematopoietic progenitors in this species. We found that LTBMCs initiated with a single marrow inoculum produced myeloid colony progenitors continuously for at least 7 weeks. The sites of myelopoietic activity in these cultures were uniquely restricted to isolated, morphologically diverse germinal centers rather than more disperse cobblestone patches. We also used the progenitor assay to screen several human and murine recombinant cytokines for cross-reactivity to swine bone marrow cells, including interleukin-3 (IL-3), IL-6, Il-11, granulocyte and granulocyte-macrophage colony-stimulating factors (G-CSF and GM-CSF), c-kit ligand (also called mast cell growth factor [MGF]), and erythropoietin (Epo). With the exception of human and murine IL-3, each of the cytokines tested induced swine progenitor colony formation to varying degrees, with some combinations leading to the formation of primitive multilineage and high proliferative potential colonies. Finally, in an attempt to characterize alternative sources of HSC from swine, we compared the progenitor content of adult and juvenile swine bone marrow and fetal liver. The fetal liver samples were found to be highly enriched for both primitive and mature progenitors, while analysis of postnatal marrow samples revealed an approximately two-fold decline in overall progenitor frequency between the ages of 10 and 20 weeks. Taken together, these studies demonstrate the development and use of in vitro culture methods for characterizing hematopoietic elements from miniature swine and suggest a hierarchy of progenitor cell content in various hematopoietic tissues from the large animal model.