The pharmacokinetics, biodistribution and efficacy of the radiopharmaceutical 131I-meta-iodobenzylguanidine (131I-mIBG) were determined in murine xenografts of two human neuroblastoma cell lines, SK-N-SH and SK-N-BE(2c). These lines have similar capacities in vitro for active uptake of 131I-mIBG, but different radiobiological characteristics. Groups of four mice were killed after injection of 131I-mIBG, and retained radioactivity in the tumour and normal tissues was measured at 8, 16, 24 and 48 h. Within each type there was heterogeneity of tumour uptake, although average values were similar for both. The per cent injected dose per gram of tumour retained at 24 h was (mean and 95% confidence interval) 0.95 (0.67-1.23) for SK-N-SH and 0.76 (0.47-1.05) for SK-N-BE(2c). The growth of tumours in groups of seven animals following injection of 35, 70 or 105 MBq 131I-mIBG was compared with that of controls. The specific regrowth delay (median and 95% confidence intervals) caused by 105 MBq 131I-mIBG was 4.2 (0.9-5.9) in SK-N-SH and 5.6 (0-11.3) in SK-N-BE(2c) bearing mice. SK-N-BE(2c) xenografts were significantly more sensitive to external beam irradiation than SK-N-SH xenografts.