Comparisons were made with age on phagocytosis, chemotaxis, and esterase staining of autoimmune-resistant and autoimmune-susceptible mouse strains. Consistent increases of each parameter occurred with age. Autoimmune strains generally showed less change with age than autoimmune-resistant mice and the changes with aging were less consistent. These findings suggest that phagocytic cells may play an important role in the autoimmunities or the lymphoproliferative processes that occur in autoimmune-prone mice and/or in loss of immune function with aging.