The CD 34 antigen is a glycoprotein found on the surface of hematopoietic stem cells and early committed progenitors. The CE 34 stem cells from 14 samples of bone marrow, cord blood or leucapheresis were isolated using a positive selection procedure involving an anti CD 34 biotinylated monoclonal antibody and an avidin immunoabsorption device. Results showed that in 60 percent of samples, the positively-selected fractions contained more than 70 percent CD 34 cells. Concentration in CFU-GM and BFU-E progenitors increased 15 and 26 fold respectively in the CE 34 enriched samples. Long-term culture of two samples demonstrated that nearly all of the most immature progenitors were recovered in the procedure. These preliminary results showed that the positive selection technique of CD 34 hematopoietic stem cells is now available for use in autologous or allogeneic hematopoietic stem cell transplantation.