Whereas the expression of foreign genes in mammalian cells usually proves successful, the purification of gene products is often a difficult and time-consuming process. The availability of monoclonal antibodies to the foreign protein can greatly assist in small scale purification, but where antibodies are not available, alternatives have to be sought One useful approach involves the fusion of the foreign gene adjacent to a gene segment encoding an antibody heavy chain variable region (1 ). By transfection of this construct into a cell line producing a compatible light chain or by cotransfection of the fusion product with a light chain gene, an antibody-like molecule can be produced and purified using the corresponding antigen.