The mouse monoclonal antibody (MAb) AUAI, when applied on LoVo tumour sections, reacts by staining all tumour cells, on their cell surfaces. To investigate the accessibility of these sites to antibody when the tumour is present as a solid mass in vivo, subcutaneous xenografts of LoVo were first prepared in nude mice. The mice were then injected intravenously with either 125I‐labelled AUAI, 125I AUAI F(ab')2 or with 125I‐labelled HMFG2 (negative control antibody). Animals were killed at various time intervals. Gross and micro‐autoradiography as well as immunohistochemistry were performed on tissue samples of tumour and control organs. The in vivo injected antibody, in contrast to that studied in vitro, was localized only, as detected by auto radiography, on a thin layer of tumour cells adjacent to the vascularized stroma. On microscopically small tumour islands the antibody penetration was complete. Most of the radioactivity was on the cell surfaces, as seen on in vitro immunostaining. With intact antibody, similar autoradiographic results were obtained at days 1,3 and 6. With F(ab')2 fragments there was deeper penetration into the tumour at days 1 and 3, though less radioactivity was found; by day 6 the activity had greatly decreased. Radioactivity in the control organs was limited to the blood pool. Negative control antibody HMFG2 showed no localization on the tumour cells. These results were not due to differences in antigenic expression of the tumour cells but reflect the problem of accessibility of antigenic sites in vivo.