The recombination-activating genes RAG-1 and RAG-2 are required for V(D)J DNA rearrangements at loci for immunoglobulin and T cell receptor genes. We isolated the human RAG-2 gene and determined its nucleotide sequence. Mapping analysis of RAG-1 and RAG-2 genes on human chromosomes by fluorescence in situ hybridization indicated that the genes are located on chromosome 11p13-p12. RAG-1 and RAG-2 do not seem to be linked to any of the primary immunodeficiencies for which defective genes have already been mapped.