V(D)J (V, variable; D, diversity; J, joining) combination of immunoglobulin (Ig) genes established in premature B cells has been thought to be conserved throughout differentiation at mature stages. However, germinal center (GC) B cells have been shown to reexpress recombination-activating gene (RAG)-1 and RAG-2 proteins in immunized mice. Here, we present several lines of evidence indicating that RAG proteins thus induced are functional as the V(D)J recombinase. DNA excision product reflecting Vλ1 to Jλ1 rearrangement was generated in parallel with the expression of RAG genes in mature mouse B cells that were activated in vitro with LPS and IL-4. Similar λ chain gene rearrangement was observed in the draining lymph node of immunized mice. Further, B cells that underwent λ gene rearrangement were shown by in situ PCR to be localized within GCs. Thus, secondary rearrangement of Ig genes (receptor editing) can occur in mature B cells.