The number of white blood cells, the morphological changes of the peripheral lymphocytes and some immunological variables were examined in 118 centenarians and in 499 healthy subjects of 60-89 years of age. There was no change in the absolute number of leucocytes, lymphocytes, or T or B cells with age. The occurrence of a myelin-like structure in the mitochondria and the presence of lipofuscin in the cytoplasm of lymphocytes increased with age and showed a correlation with the absolute number of T cells. A correlation could be observed between the absolute number of B cells, serum IgG level and the presence of antinuclear factor.