We studied the phenotypic and functional properties of colony-forming cells (CFCs), primitive long-term culture initiating cells (LTC-ICs) and lymphoid precursors present in human fetal liver (FL) and compared these with their adult bone marrow (BM) counterparts. FL (7-14-week) cells were selected by fluorescence-activated cell sorting based on increasing CD34 antigen expression (34+, 34++, or 34 ), CD38 antigen expression (CD34++/ CD38+, or CD38-), and HLA-DR antigen expression (CD34++/ HLA-DR+ or HLA-DR-). 13 +/- 0.6% of FL CD34-positive cells were 34 . Significantly more FL CD34++/ cells were CD38- (49 +/- 2.4%) and HLA-DR-(72 +/- 6.7%) than BM CD34++ cells (6.8 +/- 0.7% CD38- and 13.3 +/- 3.2% HLA-DR-). FL and BM CFCs were CD34+/++, CD38+, and HLA-DR+. However, significantly more FL CFCs were erythroid (40%) than adult BM CFCs (15%), and FL colonies were larger (8111 +/- 738 cells/CFC) than BM colonies (3466 +/- 272 cells/CFC, p < 0.001). As is seen in adult BM, FL LTC-ICs were CD34++/ CD38-. In contrast to BM LTC-ICs, FL LTC-ICs were almost exclusively CD34++/ HLA-DR+. In addition, a single FL LTC-IC gave rise to >30 CFCs at 5 weeks compared with only 5 +/- 0.9 CFCs per LTC-IC from BM. Finally, we demonstrate that the FL CD34++/ /CD38-/HLA-DR+ population, which contains 3.7% LTC-ICs, also contains primitive lymphoid progenitors capable of differentiating into natural killer (NK) cells. In conclusion, the phenotype of primitive human FL progenitors such as LTC-IC and primitive NK progenitors is CD34++/ /CD38-/HLA-DR+, suggesting that this population may contain FL hematopoietic stem cells. The phenotypic characterization of FL primitive LTC-ICs and NK progenitors will facilitate further studies of the functional properties of these progenitors.