A mAb called V65 was raised to a CD3+, TCR-alpha/beta- rat/mouse T cell hybrid that selectively reacts with all CD3+, TCR-alpha/beta- rat lymphocytes. Both anti-CD3 and V65 precipitate a 48- to 50-kDa heterodimeric protein from digitonin-lysed surface-iodinated cells. V65+ but not V65- T cells and T cell hybridoma cells express TCR-gamma mRNA. Together, these results show that V65 detects a constant determinant of the rat TCR-gamma/delta. In the presence of either IL-2 or IL-4, V65 stimulates proliferation in peripheral rat gamma/delta T cells. Approximately 90% of gamma/delta T cells from peripheral lymphoid organs have the same cell surface phenotype as thymus-derived MHC class I-restricted alpha/beta T cells, i.e., they are CD4- but express the CD8 alpha/beta heterodimer together with CD2 and CD5. In contrast, gamma/delta T cells from the epithelium of the small intestine lack CD2, CD4, and CD5 and express CD8 alpha only. Finally, V65 directly identifies a dense network of dendritic cells in the epidermis as gamma/delta T cells. These dendritic epidermal T cells are absent from athymic rats, indicating that like their mouse counterparts, they are thymus dependent.