The total lymphocyte count and the different lymphocyte subpopulations were determined in peripheral blood of healthy probands aged 20 to 90 years. With increasing age there is a slow decrease of the total lymphocyte count and of the subpopulations. This was most evident in the decrease in thymus dependent lymphocytes (T cells) which carry cellular immunity. These results support the assumption that with increasing age cellular immunity is clearly reduced and may possibly be associated with an increased incidence of neoplasia, infection, and auto-immune disease. Estimations of lymphocytes and their subpopulations in peripheral blood allow some conclusions as to the immunological resistance which is largely dependent on these cells. Estimation of the number of circulating lymphocytes in relation to age is thus indispensable for the assessment of the immunological reactivity of patients.