Mice homozygous for the mutation scid (scid mice) are severely immunodeficient and generally lack detectable numbers of pre-B, B, and T cells. This condition is believed to result from a defect in the mechanism responsible for rearrangement of immunoglobulin and T-cell receptor genes in developing B and T lymphocytes. To test this hypothesis and evaluate whether scid affects only the process of gene recombination, we introduced functionally rearranged immunoglobulin genes into the scid mouse genome. As scid mice appear to contain early lymphoid cells committed to the B lineage (pro-B cells), we asked whether the introduction of an IgM heavy-chain gene alone (mu-transgenic scid mice) or both IgM heavy- and kappa light-chain genes (mu kappa-transgenic scid mice) would allow further differentiation of scid pro-B cells into pre-B and B cells. We found that normal numbers of pre-B cells appeared in the bone marrow of mu-transgenic scid mice and that both pre-B and B cells appeared in the bone marrow of mu kappa-transgenic scid mice. However, in the latter case, the number of pre-B and B cells was 2- to 3-fold less than in the controls (mu kappa-transgenic scid heterozygotes) and few, if any, B cells were detectable in the peripheral lymphoid tissues. The implications of these results for the above hypothesis are discussed.