Ex vivo expansion of peripheral blood mononuclear cells (MNCs), cultured both directly and after selection for CD34+ cells, was compared in static and continuously perfused cultures containing interleukin (IL)-3, IL-6, granulocyte colony-stimulating factor (G-CSF), and stem cell factor (SCF). Cultures inoculated with either MNCs or CD34+ cells produced cells that were remarkably similar after 10 days of culture, as evidence by cell morphology, expression of CD34, CD33, CD15, and CD11b, and the fractions of cells giving rise to colony-forming units granulocyte-monocyte (CFU-GM) and long-term culture-initiating cells (LTC-IC). Static and perfusion cultures gave similar average total cells and CFU-GM expansions for both MNC and CD34+ cell cultures. However, those samples that performed poorly in static culture performed at near-normal levels in perfusion. In addition, perfusion supported higher LTC-IC numbers for both MNC and CD34+ cell cultures. While total cell expansion was about ten times greater in CD34+ cell cultures (approximately 100-fold), CFU-GM expansion (approximately 20-fold) was similar for both MNC and CD34+ cell cultures. The similar distribution of cell types produced in MNC and CD34+ cell cultures allows direct comparison of total and colony-forming cell production. After 15 days in perfusion, MNC cultures produced 1.5-, 2.6-, and 2.1-fold more total cells, CFU-GM, and LTC-IC, respectively, than the same sample selected and cultured as CD34+ cells. Even if the CD34+ selection process was 100% efficient, CFU-GM production would be 1.5-fold greater for MNCs than for CD34+ cells.