Epstein-Barr virus nuclear antigen 2 (EBNA-2) plays an essential role in primary B-lymphocyte growth transformation. EBNA-2 is an acidic transcriptional transactivator that is brought to virus and cell EBNA-2 response elements by interaction with a factor that recognizes the double-stranded sequence MNYYGTGGGAA, where M is A or C, N is any nucleotide, and Y is a pyrimidine. A 63-kDa protein that recognizes this DNA sequence has now been purified by S-Sepharose and oligonucleotide affinity chromatography. p63 peptide sequence is identical to the predicted amino acid sequence for the human J kappa immunoglobulin recombination signal binding protein. Purified or recombinant in vitro-translated J kappa binds to the MNYYGTGGGAA EBNA-2 response element sequence and interacts with EBNA-2. Surprisingly, J kappa does not bind to the J kappa 1 heptamer recombination signal sequence (CACTGTG), and its prior identification as a heptamer binding protein was most likely due to the addition of a BamHI restriction site to the native heptamer creating a near EBNA-2 response element consensus (CACTGTGGGAT).