SUMMARYSo far all studies on the murine ageing process have been conducted on virgin mice. Immune ageing may be influenced by sex hormone differences related to sex or pregnancies. The aim of this study was to investigate whether pregnancies and gender influence the cell changes observed during ageing in a peripheral lymphoid compartment of C57Bl/6 mice. Using flow cytometry, changes in (Thy1:2 þ ) T cell, (B220 þ ) B cell and (CD11b/Mac-1) macrophage spleen populations were monitored in 2, 8 (3 months after last pregnancy) 15 and 23-month-old mice including males, virgin and multiparous females. The development of naive (CD44 low ), memory (CD44 high ), activated/memory (MEL-14, CD62L) cells were investigated in CD4 þ and CD8 þ T cell subsets. Both short term (at 8 months) and long term (at 15 and 23 months) effects of multiparity were obvious in the lymphocyte/macrophage population changes associated with the ageing process. Short-term effects included delayed appearance of CD4 þ CD44 high memory lymphocytes and increased numbers of both CD4 þ MEL-14 low activated/memory cells and Mac-1 þ macrophages when compared with virgin control mice. Later effects of multiparity were increased CD8a dull populations and increased T/B cell ratios and the ratio of memory to naive CD4. A sex effect was noticed: males exhibited lower Mac-1 þ levels and memory/naive ratio in CD4 þ subset than virgin females throughout life. These results suggest that gender and/or pregnancies affect the age-related distribution of lymphoid and macrophage cell populations in the spleen of C57Bl/6 mice.