In accordance with their lack of MHC restriction, most mouse and human gamma delta T cells express neither the CD4 nor CD8(alpha beta) coreceptor. In striking contrast, up to 80% of splenic rat gamma delta T cells express the CD8alpha beta isoform of CD8, which for the alpha beta T cell subset serves as a marker for MHC class I-restricted cells. We compared CD8 on alpha beta and gamma delta T cells with regard to co-stimulatory function and correlation of CD8 expression with TCRDV usage and CDR3delta length. In both subsets, CD8 acted as a co-stimulatory molecule in vitro and was found to bind the kinase lck efficiently. No differences between the CDR3delta length spectra of CD8+ and CD8- gamma delta T cells or between unselected thymic and peripheral gamma delta T cells were found. As seen in man and mice, CDR3delta were rather long, a structural feature which can be expected to interfere with an alpha beta TCR-like mode of MHC class I binding. In summary, CD8 expressed by rat gamma delta T cells is a molecule with the potential to act as a coreceptor, but its expression gives no indication for antigen recognition analogous to that of MHC class I-restricted alpha beta T cells.