Monoclonal antibody Po66, produced by immunization against a patient's lung squamous cell carcinoma was found suitable for the scintigraphic detection of human tumours. Surprisingly, the cellular antigen recognized by Po66 was abundant in the cytoplasm of tumour cells but could not be detected on the surface membrane. In the present work the biodistribution of radiolabelled Po66 and of an unrelated immunoglobulin were studied comparatively after intravenous injection into nude mice bearing lung squamous cell carcinoma grafts. Radioactivity distribution among mouse organs and tumour was analysed by gamma counting and autohistoradiography. After injection, radiolabelled Po66 decreased rapidly from the blood in tumour-bearing animals whereas, in controls, it remained at a level comparable to that of the unrelated immunoglobulin. The antibody seemed slowly trapped by the tumour and, 12 days after its injection, distribution ratios between tumour and mouse organs reached values of 20-30 as against 1 in animals injected with the non-specific immunoglobulin. Autohistoradiographic investigations in the tumour confirmed the slow diffusion rate of the antibody, which remained in the vascular spaces up to the 24th hour after injection and diffused afterwards throughout the clusters of tumor cells. Furthermore, radioactivity was detected in cells which, unexpectedly, seemed morphologically unaltered. These cells, the viability of which remains to be determined, were predominant in the central area of the tumours. The results presented constitute new evidence of the ability of an in vivo injected monoclonal antibody to reach a cytoplasmic target inside non-necrotic cells and suggest that the cells permeable to the antibody might be in defective nutritional conditions.