The frequency of gamma delta + T cells in the peripheral blood of 26 Turkish, 24 Swedish, 35 Japanese and 14 "Asian' (non-Japanese) healthy blood donors and healthy volunteers were investigated by flow cytometry. In the Turkish group, 9.3% (median value) of the CD3+ peripheral blood T cells expressed the gamma delta T cell receptor. A similar level of gamma delta + T cells was found in the non-Japanese "Asian' healthy volunteers (9.2%), while significantly lower values were detected in the Swedish (4.2%) and Japanese (4.5%) groups. These dramatic differences in normally occurring gamma delta + T cells in different groups of healthy individuals were further reflected by a low incidence of > 10% gamma delta + T cells in the Swedish (0/24) and Japanese (6/35) groups compared to the Turkish (12/26) and "Asian' (5/14) groups. The described gamma delta + T cell differences between distinct ethnic groups are thus likely to be a consequence of environmental factors, but additional genetic influences cannot be ruled out. The present study demonstrates the potential importance of the ethnic origin and environmental history of subjects examined in studies of gamma delta + T cells-disease relations.