Autologous bone marrow transplantation (ABMT) offers a therapeutic alternative for children with poor prognosis acute lymphoblastic leukemia (ALL) who lack an HLA-matched sibling donor. The most common cause of treatment failure after ABMT in these patients is leukemia relapse. We have developed an ex vivo autologous marrow purging program for children with ALL using an immunomagnetic method. BM purging has been performed in 37 children with ALL (31 B-lineage ALL and 6 T-lineage ALL) following an indirect method, using panels of mouse monoclonal antibodies (MAbs) directed against B or T cell antigens, Dynabeads M-450 (Dynal) coated with sheep antimouse (SAM) antibodies, and the MaxSep Magnetic Cell Separator (Baxter). Purging efficiency has been assessed by flow cytometry. Considering the limit of detection of target cells 0.1%, the median depletion was 2.0 log (range 0.8- > 2.8 log) for the B-lineage ALL and 2.7 (range 2.2- > 2, 9 log) for the T-lineage ALL patients. Twenty-seven patients have been autografted (6 in first complete remission, CR, 13 in second CR, and 8 in third or subsequent CR). Engraftment has been satisfactory in all of them, reaching levels of 500 neutrophils/mm3 and 20,000 platelets/mm3 after a median of 17 (range 12-39) and 30 (range 13-96) days post-ABMT, respectively. In summary, our results show that this immunomagnetic procedure achieves high levels of target cell depletion and can be safely applied to bone marrow purging in childhood ALL patients.