A method of producing a transgenic mouse expressing human Ig-κ light chains and human Ig-λ light chains, comprising the steps of: producing a mouse embryonic stem cell comprising a human λ light chain locus, in germline configuration, and stably integrated into the genome of the ES cell, wherein said λ light chain locus comprises the V λ gene segments from 5' of the V λ1-27 gene to 3' of the 3' λ enhancer; producing a transgenic mouse from the ES cell of step; and mating said transgenic mouse of step with a transgenic mouse whose genome comprises: (i) a stably integrated portion of a human κ light chain locus in germline configuration, wherein said portion comprises the Vκ genes starting at Vκ- B3 and ending at Vκ -OP11, with a deletion of Vκ -LP13 to Vκ -LP6; the Jκ region; the Cκ region; the κ deleting element; and the Igκ intronic and 3' enhancers; and (ii) a stably integrated portion of a human immunoglobulin heavy chain locus in germline configuration, wherein said portion comprises the V H genes starting at V H6-1 and ending at V H3-65; the D H region, the J H region, the Cμ constant region genes, the Cδ constant region genes, Cγ constant region genes, the mouse 3' enhancer, and the human intronic enhancer, wherein said Cγ constant region genes are selected from the group consisting of: human Cγ1 constant region genes, human Cγ2 constant region genes, and human Cγ4 constant region genes; (iii) homozygous inactivated endogenous immunoglobulin heavy chain loci; and (iv) homozygous inactivated endogenous immunoglobulin κ light chain loci; to produce an offspring transgenic mouse whose genome comprises: (a) said human λ light chain locus, in germline configuration, and stably integrated; (b) said stably integrated portion of a human κ light chain locus in germline configuration; (c) said stably integrated portion of a human immunoglobulin heavy chain locus in germline configuration; (d) homozygous inactivated endogenous immunoglobulin heavy chain loci; and (e) homozygous inactivated endogenous immunoglobulin κ light chain loci. producing a mouse embryonic stem cell comprising a human λ light chain locus, in germline configuration, and stably integrated into the genome of the ES cell, wherein said λ light chain locus comprises the V λ gene segments from 5' of the V λ1-27 gene to 3' of the 3' λ enhancer; producing a transgenic mouse from the ES cell of step; and mating said transgenic mouse of step with a transgenic mouse whose genome comprises: (i) a stably integrated portion of a human κ light chain locus in germline configuration, wherein said portion comprises the Vκ genes starting at Vκ- B3 and ending at Vκ -OP11, with a deletion of Vκ -LP13 to Vκ -LP6; the Jκ region; the Cκ region; the κ deleting element; and the Igκ intronic and 3' enhancers; and (ii) a stably integrated portion of a human immunoglobulin heavy chain locus in germline configuration, wherein said portion comprises the V H genes starting at V H6-1 and ending at V H3-65; the D H region, the J H region, the Cμ constant region genes, the Cδ constant region genes, Cγ constant region genes, the mouse 3' enhancer, and the human intronic enhancer, wherein said Cγ constant region genes are selected from the group consisting of: human Cγ1 constant region genes, human Cγ2 constant region genes, and human Cγ4 constant region genes; (iii) homozygous inactivated endogenous immunoglobulin heavy chain loci; and (iv) homozygous inactivated endogenous immunoglobulin κ light chain loci; to produce an offspring transgenic mouse whose genome comprises: (a) said human λ light chain locus, in germline configuration, and stably integrated; (b) said stably integrated portion of a human κ light chain locus in germline configuration; (c) said stably integrated portion of a human immunoglobulin heavy chain locus in germline configuration; (d) homozygous inactivated endogenous immunoglobulin heavy chain loci; and (e) homozygous inactivated endogenous immunoglobulin κ light chain loci. (i) a stably integrated portion of a human κ light chain locus in germline configuration, wherein said portion comprises the Vκ genes starting at Vκ- B3 and ending at Vκ -OP11, with a deletion of Vκ -LP13 to Vκ -LP6; the Jκ region; the Cκ region; the κ deleting element; and the Igκ intronic and 3' enhancers; and (ii) a stably integrated portion of a human immunoglobulin heavy chain locus in germline configuration, wherein said portion comprises the V H genes starting at V H6-1 and ending at V H3-65; the D H region, the J H region, the Cμ constant region genes, the Cδ constant region genes, Cγ constant region genes, the mouse 3' enhancer, and the human intronic enhancer, wherein said Cγ constant region genes are selected from the group consisting of: human Cγ1 constant region genes, human Cγ2 constant region genes, and human Cγ4 constant region genes; (iii) homozygous inactivated endogenous immunoglobulin heavy chain loci; and (iv) homozygous inactivated endogenous immunoglobulin κ light chain loci; (a) said human λ light chain locus, in germline configuration, and stably integrated; (b) said stably integrated portion of a human κ light chain locus in germline configuration; (c) said stably integrated portion of a human immunoglobulin heavy chain locus in germline configuration; (d) homozygous inactivated endogenous immunoglobulin heavy chain loci; and (e) homozygous inactivated endogenous immunoglobulin κ light chain loci.