As a probe of whether RAG-1 and RAG-2 gene products activate other genes or form part of the recombinase itself, certain mutants of the RAG genes were assayed for their ability to activate variable-diversity-joining region [V(D)J] recombination in a plasmid substrate in fibroblasts. The results indicate that the N-terminal one-third of RAG-1, including a zinc-finger-like domain, and an acidic domain of RAG-2 are dispensable for activating V(D)J recombination in a fibroblast, although they contribute quantitatively. In contrast, deletion of the C-terminal segment of RAG-1, which has homology to a topoisomerase-like protein from yeast, abolished recombination activation. These results do not support the hypothesis that the RAG gene products are transcription factors and suggest the possibility that they are parts of the recombination machinery.