In murine studies of immune senescence cell preparations from whole organs, e.g. splenocytes, are frequently reported. However, age-related changes in spleen cell types have been poorly defined throughout the spectrum of adult age or across murine strains. The aim of this study was to use flow cytometry to more fully characterize cell types within the spleen of DBA/2 mice, a strain available from the National Institute of Aging (NIA), across the entire adult age spectrum. In advanced age, B cells comprise a greater percentage of the total splenocyte population, and there is a decline in the percentage of gammadelta+T cells in the spleen. The percentage of memory CD4+T cells increases in middle age with a corresponding decrease in the percentage of naïve CD4+T cells. Expression of the co-stimulatory molecule CD28 on splenic CD4+ and CD8+T cells increases with age, but CD28 expression on peripheral blood T cells does not change. The senescence marker p16INK4a increases in B cells and CD8+T cells within the spleen and can be measured by flow cytometry.