We determined whether human marrow cells that directly form colonies in vitro could be distinguished from cells that generate or become CFC only after LTMC in the presence of irradiated marrow stromal cells. In previous studies, an anti-CD33 antibody, L4F3, and complement (C') were used to lyse nearly all CFC in marrow, and the remaining cells generated CFC in LTMC. In the present studies, marrow cells were treated with L4F3 + C' and the remaining CD33- cells were separated into CD34+ and CD34- populations and placed in LTMC. Only the CD34+ cells were found to generate significant numbers of CFC. To compare the CD33-CD34+ and CD33+CD34+ cells, we isolated each cell population using two-color FACS. Only LTMCs of the CD33-CD34+ cells generated CFC for greater than 5 wk. In contrast, cells that expressed both the CD33 and CD34 antigens, which contained most of the CFC, generated few CFC in LTMC. Fractionation of marrow cells based on right angle and forward light scattering suggested that precursors for CFC have low right angle and low forward light scattering properties. The CD33-CD34+ marrow cells were therefore further fractionated based on light scatter characteristics. Cells with low right angle and low forward light scatter formed few or no colonies on direct culture, yet generated greater numbers of CFC after 4 wk of LTMC than did cells with low right angle and high forward light scatter. Most (87-98%) CFC generated in the LTMCs that were initiated with CD33-CD34+ cells were found to express the CD33 antigen. Thus, hematopoietic progenitors with differing proliferative and differentiative potentials can be directly separated on the basis of their expression of CD33 and CD34 cell surface antigens and their light scatter properties.