The ferritin concentration in peripheral blood lymphocyte extracts was measured in 10 normal subjects, 7 patients with homozygous beta-thalassaemia, and 5 patients with iron-deficiency anaemia. The mean intracellular ferritin content was found increased in beta-thalassaemia and reduced in iron-deficient patients. Incubation of mononuclear cells in phytohaemagglutinin medium led to an increase of DNA synthesis concomitant with an increased number of lymphocytes bearing transferrin receptor and interleukin-2 receptor as measured by immunofluorescent technique. Although there was an immunological impairment of lymphocytes in patients with either iron depletion or iron loading compared to normal subjects, their ability to express transferrin receptor and interleukin-2 receptor on their cell surface was normal.