The CD34 antigen is a glycosilated transmembrane protein with a molecular weight of 105-120 kDs, whose molecular function is still unknown. At present different epitopes of this antigen are recognized by more than 20 monoclonal antibodies. By flow cytometry is quite simple to identify and enumerate the CD34+ cells, present in physiological conditions on 1-3% of normal bone marrow, 0,1-0,5% of cord blood and 0,001-0,01% of peripheral blood cells. The concomitant expression of other monoclonal antibodies allows the identification of different subsets, lineage negative or already lineage "committed", so that CD34+ cells represent an heterogeneous population with only a small number of undifferentiated progenitors. The number of circulating progenitors has highly increased in peripheral blood for few hours during the fast hematopoietic recovery after high dose chemotherapy. Growth factors are able to mobilize CD34+ cells if used alone in a short treatment schedule and the effect is amplified by combining growth factors with chemotherapy. With this treatment CD34+ cells can increase in peripheral blood up to 100-1000 fold the baseline concentration. Collection of large scale of peripheral stem cells is now possible using different models of continuous-flow blood cell separators. The vast majority of the cells harvested by apheresis are constituted by "committed" elements, myeloid peroxidase positive cells (40%), T lymphocytes (30%), monocytes (20%), B lymphocytes (1-2%), the CD34+ representing not more than 3-4% of the total cells collected. The main biological characteristic of the CD34+ cells is the capacity to reconstitute the myelo and lymphopoietic system after a myeloablative treatment. For this reason in the last few years there has been an increasing interest in using these particular stem cells in many clinical settings. Peripheral blood autografting is widely used in a large number of trials for the treatment of chemosensitive tumors. At present peripheral blood allogeneic transplants have been done in a number of patients sufficient to conclude that it is safe and able to give rise to a sustained marrow engraftment. Moreover, due to the fact that circulating stem cells are a mixture of indifferentiated progenitors and "committed" cells, the hematopoietic recovery is significantly faster both in autologous and allogeneic transplant setting. The increasing use of peripheral blood stem cells for autografting has raised the problem of tumoral contamination. The role of reinfused tumoral cells in promoting the relapse had been proved in the past. Attempts to "purge" the bone marrow of patients affected by low-grade non-Hodgkin lymphoma were done several years ago at Dana Farber Institute, strongly suggesting the importance of tumor cells left in the inoculum in modifying the prognosis. In certain tumors, such as myeloma for example, using a PCR based method, the contamination was found in all the aphereses tested. Similar data were found in samples derived from advanced breast cancer or small-cell lung cancer patients. These findings have brought to the development of different systems of stem cell "purging" or CD34+ positive selection. At present at least two or three different methods are available on the market for small and large scale bone marrow or peripheral blood stem cell processing. The ongoing trials will clarify the clinical utility. In the end the availability of large amount of enriched CD34+ cells have suggested to several investigators a possible target for gene therapy. The first data seem to suggest this is a good way to pursue, even if a clinical application remains still far from being satisfying.