Radioiodinated m-iodobenzylguanidine (MIBG) has been used with only limited success for the treatment of neural crest tumors including neuroblastoma. Use of an MIBG analogue labeled with 211At could be advantageous because of the shorter range and higher linear energy transfer of its alpha-particle emissions compared with the beta-particles emitted by 131I. The potential utility of m-[211At]astatobenzylguanidine for the treatment of neuroblastoma was investigated in vitro using 3 human neuroblastoma cell lines known to take up MIBG [SK-N-SH, SK-N-BE(2C), and SK-SY5Y] and a control line lacking MIBG uptake (SK-N-MC). Maximum binding of m-[211At]astatobenzylguanidine ([211At] MABG) to 5 x 10(5) cells after a 2-h incubation ranged from 61% for SK-N-SH to 1% for SK-N-MC. Using a limiting dilution clonogenic assay, the cytotoxicity for SK-N-SH cells of [211At]MABG was compared with [211At]astatide and no-carrier-added [131I]MIBG. A D0 of 5.8 nCi/ml was calculated for [211At]MABG compared with 482 nCi/ml for [211At] astatide, indicating a more than 80-fold enhanced cytotoxicity for the specifically targeted alpha-particles of [211At]MABG. For [211At]MABG, the D0 corresponded to only 6.4 211At atoms bound/cell compared with 9000 atoms/cell for no-carrier-added [131I]MIBG. The D0 values measured for [211At]MABG treatment of SK-SY5Y, SK-N-BE(2C), and SK-N-MC cells were 50, 5.8, and 11,043 nCi/ml, respectively, corresponding to 7.04, 6.46, and 171.79 211At atoms bound/cell. In conclusion, these results have demonstrated that [211At]MABG is considerably more cytotoxic than [131I]MIBG and that [211At]MABG could have great potential as a radiotherapeutic agent for the treatment of neuroblastoma.