Meta-iodobenzylguanidine conjugated to 131I-iodine is an effective agent for the targeted radiotherapy of tumors of neural crest origin which express the noradrenaline transporter (NAT). The therapeutic application of 131I MIBG is presently limited to the treatment of phaeochromocytoma, neuroblastoma, carcinoid and medullary thyroid carcinoma. To determine the feasibility of MIBG targeting for a wider range of tumor types, we employed plasmid-mediated transfer of the NAT gene into a human glioblastoma cell line (UVW) which does not express the NAT gene. This resulted in a 15-fold increase in uptake of MIBG by the host cells. A dose-dependent toxicity of 131I MIBG to the transfectants was demonstrated using three methods: (1) survival of clonogens derived from monolayer culture; (2) survival of clonogens derived from disaggregated multicellular spheroids; and (3) spheroid growth delay. 131I MIBG was twice as toxic to cells in spheroids compared with those in monolayers, consistent with a greater effect of radiation cross-fire (radiological bystander effect) from 131I beta-radiation in the three-dimensional tumor spheroids. The highest concentration of 131I MIBG tested (1 MBq/ml) was nontoxic to UVW control cells or spheroids transfected with the NAT gene in reverse orientation. These findings are encouraging for the development of NAT gene transfer-mediated 131I MIBG therapy.