Rearrangement and in most cases expression of the T cell rearranging genes gamma (TRG gamma) and T cell antigen receptor beta chain (TCR beta) genes were studied in 19 cases of T cell acute malignancies where the surface phenotype is representative of the different stages of thymic maturation. TCR alpha gene transcription was also studied. TRG gamma and TCR beta genes were found to be rearranged in all but one case. The TRG gamma rearrangement pattern seen in most cases is compatible with biallelic rearrangement by loop excision involving the J gamma 2 regions. The sizes of all but two rearranged bands were identical to those of the rearranged bands seen in polyclonal T lymphocytes also studied in this work. One identical-sized band was found in 11 of the 18 rearranged cases. The expression of TRG gamma mRNA (transcripts of 1.6 kilobases [kb]) was highly variable from case to case and did not correlate with the stage of differentiation of the malignant cells, the expression of the molecules CD4 and CD8, the expression and size of the transcripts of the TCR beta genes, and the transcription of TCR alpha genes. In one CD3 + case, strong expression of the TRG gamma transcripts coexisted with the exclusive presence of TCR beta mRNA of 1.0 kb. The cells from this case did not react with anti-Ti antibody and exhibited no natural killer activity. These findings are suggestive of a malignancy that may express the recently isolated CD3-TRG gamma complex.