Expression of CD45RB on CD4+or CD8+cells in combination with TCRVβusages (Vβ6, Vβ8.1, Vβ8.2, Vβ11 and Vβ7a) in normal mouse strains (BALB/c and C57BL/6) was compared with autoimmune-prone strains ((NZW×BXSB) F1 and MRL/1pr) at young and old ages. The frequencies, and also the numbers of CD45RB cells in CD4+T cells with various TcR repertoires was significantly less in the autoimmune-prone strains at old ages, while, in normal control strains, they remained unchanged. Furthermore, CD4+/CD45RB-cells are CD4highand CD62L (L-selectin).lowThese findings suggest that most T cells, especially CD4+T cells, in old W/BF1 and old MRL/1pr mice, were activated and this may reflect the elevation of autoantibodies and the progress of autoimmune status in aged autoimmune-prone mice. This will be discussed in relation to the progress of the autoimmune diseases.