Monoclonal antibody (MAb) E48 and its F(ab')2 fragment, radiolabelled with1311, were tested for tumour localisation and imaging in nude mice bearing a squamous cell carcinoma xenograft line derived from a head and neck carcinoma (HNX-HN) or from a vulva carcinoma (VX-A431). MAb IgG or F(ab')2fragments were injected in parallel and at day 1, 2, 3 and 6 or 7, mice were either scanned with a gamma camera or dissected for determination of isotope biodistribution. In HNX-HN bearing mice, E48 IgG as well as F(ab')2 showed highly specific localisation in tumour tissue. The mean tumour uptake (n = 4) expressed as the percentage of the injected dose per gram of tumour tissue (percentage ID/g) of IgG was 11. 9% at day 1 and increased to 14. 6% at day 6 whereas percentage ID/g of F(ab')2 was 7. 2% at day 1 and decreased during subsequent days. Tumour to blood ratios (T/B) at day 1 were 1. 2 for IgG and 13. 6 for F(ab')2 and reached a maximum at day 6 with values of 6. 4 and 54. 2 respectively. In VX-A431 bearing mice, only E48 F(ab')2showed preferential localisation in tumour tissue. At day 1, Percentage ID/g of IgG was 3. 7 and T/B was 0. 3, while percentage ID/g of F(ab')2 was 2. 4 and T/B was 3. 2. Percentage ID/g decreased after day 1 while T/B increased. In these experiments no preferential localisation of either isotype matched25I-labelled control IgG or F(ab')2 was observed. In F(ab')2 injected HNX-HN bearing mice as well as VX-A431 bearing mice, tumours could be visualised at day 1 and 2 without any appreciable background activity. With MAb IgG this was also possible in HNX-HN bearing mice (but not in VX-A431 bearing mice) but only at day 3 and 6. These findings suggest that the superior tumour to non-tumour ratios render the E48 F(ab')2 fragment more qualified for specific targeting of radioisotopes to tumour xenografts in this experimental setting.