The organization and expression of immunoglobulin genes were studied in a series of six hybridomas derived from the fusion of a nonproducing myeloma cell with cells from mouse fetal liver. These hybridomas, which exhibit several phenotypic characteristics of immature B lymphocytes, all have productively rearranged μ heavy chain genes and produce both the membrane and secreted forms of μ mRNA in a ratio of about 1:10. Significantly, none of the hybridomas has an unrearranged (germ line) allelic μ gene. Examination of the κ light chain genes revealed that all six of the hybridomas contain unrearranged κ loci and produce 8.4-kilobase transcripts containing κ constant region sequences. None of the five hybridomas that exhibit a μ-only phenotype contains a rearranged κ gene other than that derived from the myeloma parent. One hybridoma, which actively secretes κ immunoglobulin, contains a rearranged κ gene of fetal liver origin and synthesizes a distinctive κ mRNA precursor in addition to the 8.4-kilobase transcript. These results demonstrate that rearrangement of heavy chain immunoglobulin genes normally occurs prior to that of light chain genes and further indicate that the transcriptional competence of the κ constant region locus is established prior to the time of its rearrangement.