Expression of cell surface markers on human hematopoietic cells has provided a method for characterizing subsets of cells with distinct biological functions. This is largely due to the ability to separate highly purified subpopulations of cells for comparative analysis. Relationships between the cell surface phenotype of these subpopulations and their proliferative and differentiative capacity have been instrumental in defining the hierarchical organization of cells comprising the human hematopoietic system. The identification and isolation of human hematopoietic cells expressing AC133, combined with use of in vitro and in vivo assays, has provided novel insights into the hematopoietic progenitor and stem cell compartment in the human. More recent studies have offered evidence that AC133 expression is not limited to primitive blood cells, but also defines unique cell populations in non-hematopoietic tissues. These findings will be reviewed here in the context of human hematopoiesis and the potential role and utility of AC133 expression in the human.