Summary Radiolabelled meta-iodobenzylguanidine (MIBG) is selectively taken up by tumours of neuroendocrine origin, where its cellular localization is believed to be cytoplasmic. The radiopharmaceutical [1311]MIBG is now widely used in the treatment of neuroblastoma, but other radioconjugates of benzylguanidine have been little studied. We have investigated the cytotoxic efficacy of beta, alpha and Auger electronemitting radioconjugates in treating neuroblastoma cells grown in monolayer or spheroid culture. Using a no-carrier-added synthesis route, we produced 1231-, 1251-, 1311-and 211At-labelled benzylguanidines and compared their in vitro toxicity to the neuroblastoma cell line SK-N-BE(2c) grown in monolayer and spheroid culture. The Auger electron-emitting conjugates ([1231]MIBG and [1251]MIBG) and the alpha-emitting conjugate ([211At]MABG) were highly toxic to monolayers and small spheroids, whereas the beta-emitting conjugate [1311]MIBG was relatively ineffective. The Auger emitters were more effective than expected if the cellular localization of MIBG is cytoplasmic. As dosimetrically predicted however, [211At]MABG was found to be extremely potent in terms of both concentration of radioactivity and number of atoms ml-' administered. In contrast, the Auger electron emitters were ineffective in the treatment of larger spheroids, while the beta emitter showed greater efficacy. These findings suggest that short-range emitters would be well suited to the treatment of circulating tumour cells or small clumps, whereas beta emitters would be superior in the treatment of subclinical metastases or macroscopic tumours. These experimental results provide support for a clinical strategy of combinations ('cocktails') of radioconjugates in targeted radiotherapy.Keywords: meta-iodobenzylguanidine; neuroblastoma; tageted radiotherapy; astatine Meta-iodobenzylguanidine (MIBG) is a structural analogue of the neuroadrenergic blocking drugs bretylium and guanethidine. It is selectively accumulated by an active mechanism in cells of neural crest origin. Radiolabelled MIBG allows the scintigraphic imaging of neural crest tumours (Weiland et al, 1980) and ['311]MIBG is used in the treatment of neuroblastoma and phaeochromocytoma (Hartmann et al, 1987;Mastrangelo, 1987;Schwabe et al, 1987;Voute et al, 1988). Clinical experience since 1984 has demonstrated its potential, with an objective response rate of 35% in patients with progressive heavily pretreated disease (Hoefnagel, 1994). Clinical studies are now evaluating the role of ['311]MIBG at an earlier stage in therapy, either as a first line treatment or in combination with other treatment modalities (DeKraker et al, 1995;Gaze et al, 1995;Mastrangelo et al, 1995).Although many patients show beneficial responses to ['311]MIBG treatment, a significant number subsequently relapse from previously undetected tumour sites (Sisson et al, 1989). This suggests that microtumours below the limit of clinical detectability have survived ['311]MIBG therapy. A possible explanation for the rela...