SUMMARYRearrangements of immunoglobulin genes are mediated by highly conserved heptamer and nonamer recombinational signal sequence. Using a protein‐blotting procedure, a heptamer and nonamer recombinational signal sequence‐specific DNA‐binding protein(s) was examined in the nuclear extracts from lymphoid and nonlymphoid cell lines. Nuclear extracts were subjected to SDS‐polyacrylamide gel electrophoresis, and transferred by electroblotting to nitrocellulose filters. Then the filters were hybridized to *P‐labelled synthetic double‐stranded heptamer‐23bp‐nonamer or nonamer‐ l2bp‐heptamer recombinational signal sequence probes. A relatively large amount of a DNA‐binding protein(s) ofM, 115,000 for both probes was detected in the nuclear extracts from immature B and immature T cell lines. No DNA‐binding proteins were detected in a myeloma cell line. Interestingly, this DNA‐binding protein(s) might be able to recognize both heptamer and nonamer. Recombinational signal sequence‐specific DNA‐binding activity of the protein(s) and the presence of the protein(s) in a stage‐specific manner strongly suggest that the protein(s) of M, 115,000 detected here may play an important role in the re