Po66, a monoclonal antibody (MAb) directed against lung squamous cell carcinoma, has been shown, when injected intravenously, to be retained for a long time in tumors. This property encouraged trials to use Po66 as an agent of metabolic radiotherapy. The suitability of multicellular spheroids to reproduce the in vivo conditions of irradiation of tumors by Po66 was investigated in the present work. Spheroids were formed from the lung carcinoma cell line SK-MES-1. They resembled morphologically small carcinoma nodules with desmosome-like intercellular junctions at the periphery and a central necrotic core. The cells expressing the antigen recognized by Po66 had a heterogeneous distribution and were predominant in the outer layers of the cell aggregates. Spheroids were exposed to radiolabelled Po66. The MAb diffused slowly and reached a maximal incorporation after 4-12 hours incubation. A control unrelated antibody did not penetrate appreciably. Autohistoradiographic experiments suggested that the antibody accumulated in most cells expressing the antigen. The rate of MAb release from the spheroids was very low (T 1/2 = 163 h). Taken together, the data indicate that spheroids might be a relevant model to investigate the parameters controlling Po66-mediated immunoradiotherapy.