To demonstrate the precise distribution and binding of in vivo injected monoclonal antibodies on histological tumour sections, we have biotinylated our primary antibody AUA I. Biotinylated antibody was injected into nude mice bearing simultaneous subcutaneous and intraperitoneal xenografts of the human tumour LoVo. Twenty-four hours after injection, the animals were killed, tumours and control organs were removed. Antibody was demonstrated on frozen sections by incubating sections w i t h avidin biotin peroxidase complex. We compared the in vivo penetration and binding of this antibody on intraperitoneal and subcutaneous xenografts. The antibody penetration was mainly restricted t o a thin layer of tumour cells adjacent t o the vascularized stroma i n large solid subcutaneous and intraperitoneal tumours, whereas in very small intraperitoneal turnours, antibody penetration was complete. These findings were similar to our autoradiographic results. This study demonstrates that employing the biotinylated antibodies for in vivo localization studies provides superior resolution of antibody binding for morphological assessment compared t o autoradiography. Localization of a biotin label i s more precise and will permit ultra-structural studies.