There is ample evidence to show that circulating antigen can restrict effective localization of radiolabelled murine monoclonal antibodies in human tumours growing as xenografts in Nude mice. This is the result of the formation of immune complexes in the circulation. Surprisingly this effect is not seen in patients with circulating antigen, although immune complexes are formed in the circulation, and immunoscintigraphy is not compromised. Moreover, at least in some situations, the presence and level of circulating antigen correlates positively with the sensitivity of tumour imaging, and circulating antigen can be used as a criterion of patient selection for immunoscintigraphy. The reason for the dichotomy between mouse and man is unclear, and seems to be the subject of little or no current research. The introduction of chimeric or fully human monoclonal antibodies in place of murine monoclonal antibodies means that clinical situations will now mimic more precisely the animal models. The species of antibody complexing with antigen will be homologous to the patients, and this could result in handling of those complexes in a manner different from the handling of complexes with foreign (i.e. murine) antibodies. Clearly this subject warrants further investigation.