Four monoclonal antibodies reactive with the rod portion of human cytokeratin No. 8 were evaluated using experimental tumour radio-immunolocalization. Nude mice carrying human HeLa cell tumours were injected with ¹²⁵I-labelled F(ab′)2 fragments or intact IgG antibodies. Positive immunolocalization was observed with all the specific antibodies, the intact antibodies being superior to their analogous F(ab′)₂ fragments. High localization efficiency was observed despite the fact that the targets are intracellular filaments. The presence of necrotic areas in the tumours may favour the high uptake in the tumours. By computerized γ-scintigraphic evaluation of the antibody distribution in the animals, several kinetic parameters of the antibody behaviour in vivo could be determined. The biological half-lives in the tumours were determined to be > 600 h for some of the antibodies compared to a half-life of 40–80 h in non-tumour tissues. Optimal imaging was obtained as late as 10–14 days after administration of the antibody. Up to 10% of the injected antibody could be identified in the tumours, indicating a potential value in radio-immunolocalization and radio-immunotherapy using anticytokeratin antibodies.