Human 5T4 oncofoetal antigen defined by the murine 5T4 monoclonal antibody is a highly glycosylated protein expressed by trophoblast and a few specialized adult epithelia. Up-regulation of 5T4 expression in some cancers is associated with poor clinical outcome; overexpression of human 5T4 cDNA in epithelial cells can alter their morphology and motility, supporting a role for such functions in cancer and development. A murine model to study 5T4 biology and tumour immunology would be useful. The production of m5T4-specific antibodies, their use in establishing transfected cells and documenting their biological properties in vitro are described. A rat monoclonal antibody specific for mouse 5T4 molecules by ELISA, flow cytometry, immunohistochemistry and immunoprecipitation was isolated and epitope mapped. Similar to its human counterpart, murine 5T4 antigen is a 72 kDa glycoprotein (immunoprecipitation and Western blot analysis) and exhibits punctate cell surface expression, dependent upon the integrity of the actin cytoskeleton. Likewise, overexpression of autologous murine 5T4 by B16 F10 melanoma cells and A9 L fibroblasts accentuates the 5T4 phenotype, which is characterized by a spindle-like morphology, increased motility, and reduced adhesion and proliferation rate. Immunohistochemical analysis of adult mouse tissues shows a restricted pattern of expression similar to that of human 5T4 antigen. The murine 5T4 antigen-expressing cell lines and antibody reagents are now being used to explore novel immunotherapies in pre-clinical models and the biology of 5T4 in development.