Mutation of the XRCC4 gene in mammalian cells(1,2) prevents the formation of the signal and coding joints in the V(D)J recombination reaction(3), which is necessary for production of a functional immunoglobulin gene, and renders the cells highly sensitive to ionizing radiation(4). However, XRCC4 shares no sequence homology with other proteins, nor does it have a biochemical activity to indicate what its function might be(2). Here we show that DNA ligase IV (ref. 5) co-immunoprecipitates with XRCC4 and that these two proteins specifically interact with one another in a yeast two-hybrid system. Ligation of DNA double-strand breaks in a cell-free system by DNA ligase IV is increased fivefold by purified XRCC4 and seven- to eightfold when XRCC4 is co-expressed with DNA ligase IV. We conclude that the biological consequences of mutating XRCC4 are primarily due to the loss of its stimulatory effect on DNA ligase IV : the function of the XRCC4-DNA ligase IV complex may be to carry out the final steps of V(D)J recombination and joining of DNA ends.