Interaction between the RAG1 and RAG2 proteins is probably critical for V(D)J recombination. Using a coimmunoprecipitation assay, we define a large region of RAG1 (amino acids 504-1008) that is sufficient for interaction with RAG2. This region comprises the C-terminal half of the RAG1 protein, and is within the region defined as the recombinationally active core. Deletion of either of two regions of RAG1 (amino acids 504-570 or 850-1008) causes a loss of interaction with RAG2. Loss of coimmunoprecipitation is also seen with RAG1 core proteins containing deletions of smaller stretches of amino acids (amino acids 506-511 or 545-550), emphasizing the importance of this region of RAG1 in forming a complex with RAG2. A variety of other small deletion mutations within the amino acid region 504-1008 also decrease coimmunoprecipitation of RAG2 with RAG1, indicating that much or all of this region is important for complex formation.