Homozygous mutation at the scid locus in the mouse results in the aberrant rearrangement of immunoglobulin and T-cell receptor gene segments. We introduced a retroviral vector containing an inversional immunoglobulin rearrangement cassette into scid pre-B cells. Most rearrangements were accompanied by large deletions, consistent with previously characterized effects of the scid mutation. However, two cell clones were identified which contained perfect reciprocal fragments and wild-type coding joints, documenting, on a molecular level, the ability of scid pre-B cells to generate functional protein-coding domains. Subsequent rearrangement of the DGR cassette in one of these clones was accompanied by a deletion, suggesting that this cell clone had not reverted the scid mutation. Indeed, induced rearrangement of the endogenous kappa loci in these two cell clones resulted in a mixture of scid and wild-type V-Jκ joints, as assayed by a polymerase chain reaction and DNA sequencing. In addition, three immunoglobulin μ- scid pre-B cell lines showed both scid and wild-type V-Jκ joins. These experiments strongly suggest that the V(D)J recombinase activity in scid lymphoid cells is diminished but not absent, consistent with the known leakiness of the scid mutation.