Pseudomyxoma peritonei (PP) is a local slowly progressing disease with typical abdominal swelling. Treatment is uneffective and the long-term prognosis is poor. Conventional radiology provides usually only a delineation of low density area relating gelatinous masses accumulating in the peritoneal cavity. In this study, immunohistochemistry based on digital quantitative autoradiography utilizing radiolabelled monoclonal antibody B72.3 (MoAb) recognizing TAG-72 antigen on epithelial carcinomas was used for diagnosis of pseudomyxoma (7 patients). The PP patients were studied with radioiodinated I-131-labeled MoAb after intravenous (2 patients) and intraperitoneal (7 patients) injections. Radioactivities of MoAbs varied considerably in the tumors. Both intra- and extracellular staining pattern was observed by immunohistochemistry. Gamma imaging at 1, 3, 7 and 14 days after i.v. injection (2 patients) revealed targeting of all known lesions. The intraperitoneally injected MoAb (7 patients) retained long time in the peritoneal cavity, specific tumour targeting was seen up to 16 days by an antibody-SPECT, while maximum blood radioactivity was measured between 8-12 hrs. Radiolabelled B72.3 MoAb recognizing TAG-72 antigen is also present within pseudomyxoma cells. It can be used for radioimmunohistochemistry of PP. Accurate imaging of PP is possible by MoAb suggesting earlier diagnosis and more accurate location of residual disease after operations, and evaluating treatment response. Estimated tumour dose for intraperitoneal tumour (MIRD formalism) was 13 mGy/MBq. This indicates that the radioiodinated B72.3 antibody can be used for in vivo targeting and therapeutic applications of intraperitoneal pseudomyxoma.