In humans, approximately 60% of expressed immunoglobulin light chains are of the Kappa type and 40% of the Lambda type. In mice, there is almost no expression from the Lambda locus and over 95% of light chains are of Kappa type. The present invention discloses, among other things, transgenic mice carrying most of the human Ig Lambda light chain locus in their genome. The resulting mice express light chains with Kappa/Lambda ratio similar to the human ratio. Breeding of HuIg Lamda mice to Kappa-deficient mice also is described, as well as the generation of human monoclonal antibodies from transgenic mice with human Ig Lambda locus.