The radiobiological effect of 131I radiolabeled recombinant human epidermal growth factor (131I-rhEGF) on nude mice with human breast cancer was assessed in this study. The tissue mainly uptaking 131I-rhEGF was found by tissue distribution assay in mice. The radiation breakdown of the tissue greatly collecting 131I-rhEGF was examined by biochemical test and biopsy in nude mice with human breast cancer. The tissue distribution assay of 131I-rhEGF in mice showed that 131I-rhEGF greatly accumulated in kidney, liver, spleen and blood. The biochemical test and biopsy revealed that 131I -rhEGF injected twice (dosing once is analogous to 14.58 GBq in a person with 50 kg, once every 14 days) had an effective killing effect on tumor but had no effect of radiation breakdown on kidney, liver,spleen and blood-cell forming tissue in mice with human breast cancer. Therefore, 131I-rhEGF is a drug unharmful to normal tissues in the course of the receptor-mediated target radiotherapy for breast cancer.