Rare Ig and TCR coding joints can be isolated from mice that have a targeted deletion in the gene encoding the 86-kDa subunit of the Ku heterodimer, the regulatory subunit of the DNA-dependent protein kinase (DNA-PK). However in the coding joints isolated from Ku86−/− animals, there is an extreme paucity of N regions (the random nucleotides added during V (D) J recombination by the enzyme TdT). This finding is consistent with a decreased frequency of coding joints containing N regions isolated from CB-17 SCID mice that express a truncated form of the catalytic subunit of the DNA-PK (DNA-PK CS). This finding suggests an unexpected role for DNA-PK in addition of N nucleotides to coding ends during V (D) J recombination. In this report, we establish that TdT forms a stable complex with DNA-PK. Furthermore, we show that DNA-PK modulates TdT activity in vitro by limiting both the length and composition of nucleotide additions.