We have examined the arrangement of immunoglobulin heavy chain constant (CH) and joining (JH) region genes in murine T cell hybrid lines and in T lymphomas. CH genes derived from both parental cell types were present in all hybrids for which polymorphism in sequences flanking CH genes permitted us to distinguish parental CH genes. All T lymphomas and T cell hybrids retained the C alpha gene in germ-line configuration and all but one cell line had germ-line C mu genes. Novel DNA fragments reactive with JH probes were observed in six of nine T cell hybrids, as well as in two T lymphomas, WEHI7.1 and YAC-1, but not in the fusion parent, BW5147. No RNA homologous to C gamma 2b, C alpha, or lambda genes was detected in any of the T cell lines. T cell lines contained poly(A)+ RNA homologous to a C mu cDNA probe. More importantly, in several cell lines the C mu RNAs were associated with membrane-bound polyribosomes. These results suggest that both JH rearrangements and C mu RNA production occur in at least some mature, antigen-specific T cells. They may therefore reflect events in normal T cell development and function related to those involved in the generation of the T receptor for antigen.