The lymphocyte transformation test with phytohemagglutinin as mito-gen estimated according to the incorporation of 2-14C-thymidine in DNA was used as an indicator of cell-mediated reactivity in 53 healthy subjects. Three age groups were examined: up to 20 years (21 subjects), 21–40 years (10 subjects) and over 70 years (22 subjects). The responsiveness of lymphocytes decreased significantly with age. In the highest age group 12 pathologically low values were found.