Monoclonal antibody (MAb) E48 reacts with a 22 kD antigen exclusively expressed in squamous and transitional epithelia and their neoplastic counterparts. Radiolabelled with99mTc, MAb E48 is capable of targeting metastatic and recurrent disease in patients with head and neck cancer. In this study, the capacity of131I-labelled MAb E48 to eradicate xenografts of human squamous cell carcinoma of the head and neck (HNSCC) in nude mice was examined. Experimental groups received a single i.v. bolus injection of 400μCi MAb E48 IgG (number of mice (n = 6, number of tumours (t) = 9) or 800μCi MAb E48 IgG (n) = 5, t = 7), whereas control groups received either diluent (n = 3, t = 5), unlabelled MAb E48 IgG (n = 4, t = 5) or 800μCi131I-labelled isotype-matched control MAb (n = 6, t = 9). A 4.1-fold increase in the median tumour volume doubling time and regression of two out of ten tumours (20%) was observed in mice treated with 400μCi. In mice treated with 800μCi. In mice treated with 800μCi, two out of seven tumours (29%) showed complete remission without regrowth during follow-up (>3 months). Median tumour volume doubling time in the remaining five tumours was increased 7.8-fold. No antitumour effects were observed in mice injected with diluent, unlabelled MAb E48 or131I-labelled control MAb. In the same xenograft model, chemotherapy with doxorubicin, 5-fluorouracil, cisplatin, bleomycin, methotrexate or 2',2'-difluorodeoxycytidine yielded a less profound effect on tumour volume doubling time. Increases in tumour volume doubling time with these chemotherapeutic agents were 4, 2.2, 2.1, 1.7, 0, and 2.6 respectively. Moreover, no cures were observed with any of these chemotherapeutic agents. From the tissue distribution of 800μCi MAb E48, the absorbed cumulative radiation doses of tumour and various organs were calculated using the trapezoid integration method for the area under the curve. To tumour xenografts, 12,170cGy was delivered, blood received 2,984cGy, whereas in every other tissue the accumulated dose was less than 6% of the dose delivered to tumour. These data, describing the first radiolabelled MAb with therapeutic efficacy against HNSCC, suggest radioimmunotherapy with MAb E48 to be a potential therapeutic modality for the treatment of head and neck cancer.