CD34+ is a heavily glycosylated surface antigen which is preferentially expressed on haemopoietic stem/progenitor cells. No definitive function has been attributed to CD34+, but it appears to play a role in cell to cell adhesion and may be involved in signal transduction to regulate the expression of other haemopoiesis-associated genes. A number of monoclonal antibodies to CD34+ have been raised and these have allowed the identification and characterization of a whole range of haemopoietic progenitor cells. CD34+ is expressed most strongly on the most primitive cells and is progressively lost as cells differentiate. The restricted expression of CD34+ to haemopoietic stem/progenitor cells has been exploited for transplantation studies. Several techniques have been developed to select cells expressing CD34+ from haemopoietic tissues. Successful sustained engraftment can be achieved using such positively selected cells. Alternatively, CD34+ cells may be expanded in vitro by incubation with synergistic cytokine combinations before being re-infused. An exciting new development has been the use of purified populations of CD34+ cells as the targets for gene marking and gene therapy protocols.