Peripheral blood stem cells (PBSCs) are being used as an alternative to autologous marrow rescue for hematopoietic reconstitution after high-dose chemotherapy in patients with neuroblastoma and other solid malignancies. Use of PBSCs is preferred by some because of the belief that there is less risk of tumor contamination. Because tumor stem cell contamination is thought to be one contributing cause of relapse after myeloablative therapy and autologous reconstitution, we examined the potential risk of reinfusing circulating neuroblastoma cells by in vitro evaluation of their clonogenicity. Immunocytologic and tumor cell clonogenic analyses were performed on 74 blood samples obtained from 56 children with advanced-stage neuroblastoma. Concurrently drawn bone marrow specimens were evaluated in 30 instances. Circulating neoplastic cells were detected in 19 of 74 (26%) for all specimens and by immunologic techniques (26%). Using a clonogenic assay, 13 grew identifiable tumor colonies. Comparing results with the two techniques showed tumor colony growth in 10 of the 19 positive specimens by immunocytology. However, 3 of 53 samples (6%) that were negative by immunocytology were positive by the clonogenic assay. Of the 11 positive blood samples, 9 concurrent marrows contained neuroblastoma cells; of the 19 negative blood specimens, 3 concurrent marrows had metastatic disease. We conclude that circulating neuroblastoma cells are present in peripheral blood and have clonogenic properties in vitro. This supports the view that tumor cell contamination may well be one cause of relapse after autologous reconstitution. Consequently, PBSC collections should also undergo meticulous monitoring for tumor contamination before autologous reinfusion.