Tumor contamination of autologous peripheral blood stem/progenitor cell grafts occurs in a substantial proportion of high-risk breast cancer patients, and the possibility that such contamination may contribute to relapse has focused attention on methods for removal of the contaminating cells prior to transplantation. One such approach is positive selection of CD34+ cells. A fully automated immunomagnetic cell selection system has recently been introduced to facilitate the positive selection process. A multicenter randomized clinical trial was designed to evaluate the capacity of CD34+ cells isolated using the fully automated system to support prompt hematopoietic reconstitution following high-dose chemotherapy in high-risk breast cancer patients, as well as to assess the safety and tolerability of the CD34+ cell transplants. In recipients of isolated CD34+ cells, the median time to an absolute neutrophil count > or =500/microl was 10 days, a value identical to that observed in patients receiving unfractionated apheresis collections. In the isolated CD34+ cell recipients median time to a platelet count > or =20 000/microl was 12 days, compared with 10 days in the unfractionated cell group. There were no statistically significant differences between the groups in median time to neutrophil or platelet engraftment. Infusion of autologous cells was well tolerated by the study groups. There were no inter-group differences in the incidence of infections, need for platelet transfusions, or duration of hospitalization. Isolated CD34+ cells were high in purity and sufficient in number for use in autologous transplantation. The fully automated immunomagnetic cell selection system affords an efficient and time-saving option for isolation of CD34+cells to be used as autologous grafts in high-risk breast cancer patients, and the isolated CD34+ cells support undelayed hematopoietic reconstitution.