Methods have been developed to transfect immunoglobulin genes into lymphoid cells. The transfected genes are faithfully expressed, and assembly can occur both between the transfected and endogenous chains and between two transfected chains. Gene transfection can be used to reconstitute immunoglobulin molecules and to produce novel immunoglobulin molecules. These novel molecules can represent unique combinations of heavy and light chains; alternatively, by means of recombinant DNA technology, genes can be assembled in vitro, transfected, and expressed. The end products of such manipulations include chimeric molecules with variable regions joined to different isotypic constant regions; this is possible both within and between species. It is also possible to synthesize altered immunoglobulin molecules, as well as molecules having immunoglobulin sequences fused with nonimmunoglobulin sequences (for example, enzyme sequences).