The 5T4 oncofoetal antigen is a 72 kDa glycoprotein defined by a monoclonal antibody raised against human placental trophoblast and is expressed in many different carcinomas but detected only at low levels in some normal epithelia. Immunohistochemical analysis of the patterns of expression in colorectal carcinomas has indicated a significant association between the presence of the antigen in tumour cells and metastatic spread. A cDNA encoding the 5T4 molecules has been isolated and the extracellular portion contains several leucine-rich repeats which have been implicated in cellular interactions. To study the cell biological role of 5T4 molecules, murine L cells (A9 derivative) were stably transfected with 5T4 cDNA under the control of the CMV immediate-early promoter. The 5T4-expressing cells exhibited a more spindle-shaped morphology compared to the vector alone transfected cells. Confocal immunofluorescence microscopy revealed a 'polkadot' pattern of 5T4 antigen expression, heterogeneous in intensity between cells, but distributed over the entire cell surface. Transmission and scanning electron microscopy showed that the 5T4 antigen is concentrated at microvillus projections of the plasma membrane both in the transfected A9 cells and in various carcinoma cell lines. Such projections express an array of surface molecules which function in cell adhesion and motility. This association of 5T4 antigen with microvillus projections was also observed in various carcinoma cell lines. 5T4 expression in A9 cells was also associated with an altered pattern of cell division, decreased cell-substratum adhesion and increased cellular motility. These results support the hypothesis that 5T4 molecules may have a direct function in trophoblast and tumour cell invasion processes.