Fertilized mouse eggs were coinjected with mouse immunoglobulin heavy and light chain transgene constructs whose rearranged heavy chain VDJ segments and kappa light chain VJ segments were derived from the HyHEL10 plasmacytoma cell line, which produces a high affinity antibody to chicken lysozyme (commonly called hen egg lysozyme (HEL)). The heavy chain variable segments are joined to the Ighm switch region followed by the Ighm constant region and membrane terminal exons of the Igha allotype, and the light chain variable segments to the Igk constant region and 3' flanking DNA of the Igka allotype. Each construct is expressed from the promoter upstream of the leader sequence of the V-region segment. Line 4 was estimated by Southern blot analysis to carry 4-6 and 2-3 copies of the Ighm and Igk constructs, respectively.