The mechanisms of the uptake and release of m-iodobenzylguanidine (MIGB) have been studied in 5 neuroblastoma (NB) cell lines and in 4 clonal NB sublines with a homogeneous phenotype. A specific uptake system for MIBG was found in 8 of 9 NB cell lines or subpopulations. The uptake was characterized by temperature dependency, high affinity, saturability, sodium dependency, and imipramine sensitivity. The majority of NB cell lines that possessed a specific uptake system for MIBG were also able to efficiently store the incorporated drug. However, 3 NB cell lines were identified without the ability to retain high levels of MIBG, despite the presence of a specific uptake system. We also report that a clonal subline, SH-EP1, which has a nonneuroblastic phenotype, failed both MIBG uptake and retention. Conversely, the parental cell line, SK-N-SH, and the neuroblastic subline SH-SY5Y possessed both a specific uptake system and the ability to store MIBG. In addition, the induction of neuronal differentiation with retinoic acid increased the velocity of uptake and the storage efficiency for MIBG in the clonal subline SH-SY5Y. We conclude that MIBG uptake and storage should be considered to be frequent but independent neuronal functions of human NB cells.