Peripheral blood mononuclear cells were obtained from people ranging in age from newborn to 102 years old and analyzed by dual color flow cytometer in terms of number and percentage of various subsets of T cells, B cells and natural killer cells (CD3, 4, 5, 8, 11b, 19, 20, 21, 25, 29, 45RA and 56). Numbers of T cells (CD3+ or CD5+ cells) significantly declined at the 3rd decade as compared with those of younger people, stayed at a relatively constant level between the 3rd and the 7th decade and gradually declined thereafter. In T cell subsets, both CD4 and CD8 positive positive cells decreased with age, but a decrease was more pronounced in the latter, showing an age-related increase of CD4/CD8 ratio. The most interesting finding was a contrasting age-change in two subsets of CD4+ T cells; i.e. a subset of suppressor inducer T cells (CD4+CD45RA+ naive cells) decreased with age, while a subset of helper inducer T cells (CD4+CD29+ memory cells) increased with age. CD20+ B cells also decreased with age in a manner similar to that observed in T cells. Natural killer cells (CD56) showed an increase in numbers with age. The relationship between these changes in various subsets of peripheral blood leukocytes and the age-related decline in immune functions has been discussed.