The expression of the murine TCR-gamma genes was examined in a series of IL-3-dependent and growth factor-independent cell lines. All of the IL-3-dependent cell lines, but none of the IL-3-independent lines, expressed high levels of one or more of the gamma genes but did not express the TCR-beta genes. None of the cell lines expressing the gamma loci contained detectable genomic gamma gene rearrangements. Sequencing of cDNA clones from two of the cell lines demonstrated that transcription was from nonrearranged gamma loci based on the presence of sequences in the cDNAs that are found immediately 5' of the J gamma 4 and J gamma 2 genes. The expression of gamma transcripts was dependent upon IL-3 and no transcripts were detectable within 6-8 h after the removal of IL-3. Readdition of IL-3, but not granulocyte CSF, resulted in the reappearance of gamma transcripts within 30 min. The results demonstrate that IL-3 regulates the expression of nonrearranged gamma loci. Since expression is required for rearrangement, it can be hypothesized that IL-3 may influence the ability of lymphoid/myeloid progenitors to commit to the T cell lineage.