B-lymphocyte development exhibits a characteristic order of immunoglobulin gene rearrangements. Previous work has led to the hypothesis that expression of the immunoglobulin mu heavy chain induces rearrangement activity at the kappa light-chain locus. To examine this issue in more detail, we isolated five matched pairs of mu- and endogenously rearranged mu+ cell lines from the Abelson murine leukemia virus-transformed pro-B-cell line K.40. In four of the five mu+ cell lines, substantial expression of mu protein on the cell surface was observed, and this correlated with an enhanced frequency of kappa immunoglobulin gene rearrangement compared with that in the matched mu- cell lines. This increased kappa gene rearrangement frequency was not due to a general increase in the amount of V(D)J recombinase activity in the mu+ cells. Consistently, introduction of a functionally rearranged mu gene into one of the mu- pre-B-cell lines resulted in a fivefold increase in kappa gene rearrangements. In three of the four clonally matched pairs with increased kappa gene rearrangements, the increase in rearrangement frequency was not accompanied by a significant increase in germ line transcripts from the C kappa locus. However, in the fourth pair, K.40D, we observed an increase in germ line transcription of the kappa locus after expression of mu protein encoded by either an endogenously rearranged or a transfected functional heavy-chain allele. In these cells, the amount of the germ line C kappa transcript correlated with the measured frequency of rearranged kappa genes. These results support a regulated model of B-cell development in which mu protein expression in some way targets the V(D)J recombinase to the kappa gene locus.