It is thought that senescence of the immune system is responsible, at least in part, for many health problems associated with ageing. Previous studies on changes in lymphocyte composition have used flow cytometry to study peripheral blood lymphocytes (PBL's), or cells isolated from rodent tissue, and have yielded conflicting results. We have used immunohistochemistry to determine whether the B and T cells in human tissue from spleen and gut are affected by age. Areas of germinal centre, mantle zone and marginal zone of B cell follicles were measured. In addition, CD4 and CD8 T cells in T cell areas and in B cell follicles were counted. We observed a striking age-related decrease in the proportion of CD8+ T cells in the T cell zones of the spleen. This decrease was not apparent in the T cell population that occupies splenic B cell areas, or in GALT. Further differences, in CD4+ cells, were seen between T cell populations in the T cell zones and those in B cell areas. These findings highlight differences between lymphocyte populations in different lymphoid tissues, and different compartments within each tissue, which may be of importance in future studies of the ageing immune system.