A group of 1,052 C578L/6J mice — 296 males and 756 females — was kept under well defined, clean, conventional conditions from the age of 6 weeks until natural death. At regular time intervals several hematological values were examined from 50 to 60 randomly selected animals of each sex and statistically evaluated. The hematological investigations did not reveal any important sex-related differences. A significant gradual decrease of the RBC counts and hemoglobin content was observed during the lifespan. Lymphocyte and total WBC counts were subject to considerable variation. They decreased in the first two-thirds of life and increased in the last third. Neutrophils remained rather stable and exhibited an increasing trend in the higher age. The relative level of lymphocytes decreased significantly in favour of neutrophils during natural aging.