The severe combined immunodeficiency (scid) mouse mutation impairs the recombination of Ig and TCR genes. Mice homozygous for this mutation (scid mice) lack pre-B, B, and T lymphocytes. Earlier we introduced a functionally rearranged mu-heavy chain gene into the scid mouse genome and found that this resulted in the development of pre-B cells in the bone marrow of these mice; however, sIgM+ B cells were not detected. We have now investigated the growth properties and rearrangement status of Ig genes in early B-lineage cells arising in mu-transgenic scid mice. We find that the presence of a functional mu-transgene allows pro-B cells from these mice to proliferate in short-term culture with IL-7. Nevertheless, rearrangements of Ig light chain genes are not detected in the bone marrow of such mice. Furthermore, the frequency of rearrangement detected at the endogenous Ig heavy chain locus in scid pro-B and pre-B cells is reduced relative to that in wild-type cells.