The T cell-specific gamma genes in C57BL/10 (B10) mice have been analyzed. Based on the cDNA sequences of these genes from antigen-specific MHC-restricted cytotoxic T cells, we found that the repertoire of these genes is not as limited as previously postulated (8). T cells from the B10 mice express an identical copy of V gamma J gamma C gamma (V gamma 10.8A-JC gamma 10.5) transcript previously found in T cells of BALB/c mice. In addition, a potentially functional mRNA using V gamma 10.8B and newly identified J gamma and C gamma gene segments were found. The new J gamma C gamma (JC gamma 10.8) is located 5' to the inverted V gamma 10.8B in the germline DNA of both B10 and BALB/c mice. This new C gamma is only 77 and 66% homologous to the C gamma 10.5 at the nucleotide and deduced protein sequences, respectively, thus making it a potential isotype of the C gamma genes reported previously. The V gamma 5.7, J gamma 2.3 gene segments and pseudogene C gamma 7.5 found in the germline DNA of BALB/c mice are absent in B10 mice. The loss of this gamma chain pseudogene in the B10 mouse strain, and the retention of all potentially functional V gamma, J gamma, and C gamma genes with highly conserved coding sequences supports the importance of these genes.