gammadelta T cells are distinct with respect to tissue localisation, phenotype and biological functions and similarities between species are not very apparent. To elucidate local and functional heterogeneity of non-stimulated avian gammadelta T cells, the CD8-characterised gammadelta T cell subsets [CD8alpha(+high) (CD8alphaalpha(+) and CD8alphabeta(+)); CD8alpha(+dim); CD8(-)] of blood, spleen and caecum were flow cytometrically quantified and analysed for proliferation state as well as sorted for determination of immune-relevant gene expression by quantitative real-time RT-PCR. The number of avian CD8-characterised gammadelta T cell subsets differed in dependence on tissue and age of bird. Compared to blood and spleen, caecum showed the highest percentage of gammadelta T cells as well as of the CD8alpha(+high) gammadelta T cell subset in 7-week-old birds. Generally, the CD8alphabeta(+) cells significantly outnumbered the CD8alphaalpha(+) lymphocytes within the CD8alpha(+high) gammadelta T cell population of all organs. Additionally, the splenic CD8alphabeta(+) subpopulation revealed the highest proliferation activity. By RT-PCR, mRNA expression of immune-relevant genes was proved in non-stimulated gammadelta T cell subsets, but on different levels. Generally, both CD8alpha(+high) cell subsets (CD8alphaalpha(+) and CD8alphabeta(+)) of blood and spleen showed elevated expression levels for Fas ligand (FasL), XCL1 (lymphotactin) and interferon-gamma (IFNgamma) compared to the CD8alpha(-) gammadelta T cell subset. In contrast, all caecal gammadelta T cell subsets showed similar high levels of these transcripts. Notably, the CD8alphaalpha(+) cells of all locations showed unique expression of TLR4 and interleukin (IL)-2. The results demonstrated that avian gammadelta T cells are not only heterogeneous concerning their CD8 antigen characteristics and tissue localisation, but also with regard to functional features such as proliferation and mRNA expression.