AC133+ cells may represent an alternative source of transplantable haemopoietic progenitor cells to CD34+ cells. Here, we have addressed the characterization of umbilical cord blood (UCB) AC133+ cells and compared their immunophenotypic and functional features with those of UCB CD34+ cells. UCB AC133+ and CD34+ cell fractions were purified by magnetic cell sorting, analysed by flow cytometry, tested for their content in blast cell colony-forming units (CFU-Bl), erythroid and granulocyte-macrophage colony-forming units before and after expansion in the presence of various haemopoietic growth factor combinations. Median AC133+ cell yield was 62.3%, and median AC133+ population purity was 97.9%. AC133+ cells were found to contain significantly more CFU-Bl than CD34+ cells; furthermore, the replating efficiency, i.e. the number of CFU-Bl capable of generating secondary colonies, was higher in the former than in the latter cells. Both AC133+ and CD34+ cells displayed an increased ability to give rise to committed progenitors after 7-day expansion in liquid cultures. These data suggest that the AC133+ cell subset is a heterogeneous pool of immature and more differentiated cells that can be maintained and expanded in well-defined culture conditions. In comparison with CD34+ cells, UCB AC133+ cells appear to contain a higher number of early haemopoietic progenitors.