We describe a system to evaluate the function of lymphocyte-specific and generally expressed genes in the differentiation and/or function of lymphocytes. RAG-2 (recombination-activating gene 2)-deficient mice have no mature B and T lymphocytes due to the inability to initiate VDJ recombination. Blastocysts from RAG-2-deficient mice generate animals with no mature B and T cells following implantation into foster mothers. However, injection of normal ES cells into RAG-2-deficient blastocysts leads to the generation of somatic chimeras with mature B and T cells all of which derive from the injected ES cells (referred to as RAG-2-deficient blastocyst complementation). Complementation of RAG-2-deficient blastocysts with mutant ES cells heterozygous for a targeted mutation that deletes all immunoglobulin heavy-chain joining (JH) gene segments (JH+/-) also leads to generation of chimeras with normal B and T cells. However, complementation with ES cells homozygous for the JH mutation (JH-/-) generates animals with normal T cells but no B cells, due to a block in B-cell development at a very early stage. Transfection of a functionally assembled mu heavy-chain gene into the JH-/- ES cells prior to blastocyst injection rescues the JH-/- mutation and allows the generation of both mature T and mature B cells. The rescued B cells express IgM but not IgD and respond normally to bacterial lipopolysaccharide stimulation by proliferating and by secreting IgM.